Detailed chemical kinetic models for the combustion …/Detailed chemical...Detailed chemical kinetic models for the combustion of hydrocarbon fuels John M. Simmie* Department of Chemistry,

Detailed chemical kinetic models for the combustion

of hydrocarbon fuels

John M. Simmie*

Department of Chemistry, National University of Ireland, Galway, Ireland

Received 15 February 2003; accepted 11 July 2003

Abstract

The status of detailed chemical kinetic models for the intermediate to high-temperature oxidation, ignition, combustion of

hydrocarbons is reviewed in conjunction with the experiments that validate them.

All classes of hydrocarbons are covered including linear and cyclic alkanes, alkenes, alkynes as well as aromatics.

q 2003 Elsevier Ltd. All rights reserved.

Keywords: Combustion; Ignition; Oxidation; Hydrocarbons; Kinetic modeling; Kinetic modelling

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 600

1.1. Reaction mechanism design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 600

1.2. Modelling applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 601

1.3. Environments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 601

2. Alkanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 602

2.1. Methane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 602

2.1.1. Mixtures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 605

2.2. Ethane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 606

2.3. Propane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 607

2.4. Butanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 608

2.5. Pentanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 609

2.6. Hexanes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 610

2.7. Heptanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 610

2.8. Octanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 612

2.9. Decanes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 613

2.10. Higher hydrocarbons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 614

2.11. Cyclics, rings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 615

2.11.1. Three and four . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 615

2.11.2. Five. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 615

2.11.3. Six . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 615

2.11.4. Multiple rings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 616

3. Alkenes and dienes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 616

3.1. Ethene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 616

3.2. Propene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 617

3.3. Butenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 618

0360-1285/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S0360-1285(03)00060-1

Progress in Energy and Combustion Science 29 (2003) 599–634

www.elsevier.com/locate/pecs

* Tel.: þ353-91-750388; fax: þ353-91-525700.

E-mail address: [email protected] (J.M. Simmie).

http://www.elsevier.com/locate/pecs

3.4. Higher alkenes and dienes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 619

4. Alkynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 620

4.1. Ethyne. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 620

4.2. Propyne . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 620

4.3. Butynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 622

4.4. Diynes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 622

5. Aromatics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 622

5.1. Benzene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 622

5.2. Other aromatics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 623

6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 624

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 625

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 625

1. Introduction

Detailed chemical kinetic mechanisms are routinely used

to describe at the molecular level the transformation of

reactants into products, such as the combustion of methane

in air which has a deceptively simple overall summary:

CH4 þ 2O2 ¼ CO2 þ 2H2O

but proceeds, as is well known, through a large number of

elementary steps. The sets of differential equations describ-

ing the rates of formation and destruction of each species are

then numerically integrated and the computed concen-

trations of reactants, intermediates and products compared

to experiment. This procedure, known as modeling or

modelling, is widely used in studies of combustion but also

for other complex chemical phenomena, such as reactions in

the atmosphere and chemical vapour deposition [1],

astrochemistry [2] and, even petroleum cracking in

geological basins [3].

Previous reviews include a much cited, comprehensive

review of chemical kinetic modelling of hydrocarbon

combustion by Westbrook and Dryer [4], a selective view

of chemical kinetics and combustion modelling by Miller

and Kee [5], a discussion on hydrocarbon ignition by

Westbrook et al. [6], and a progress report of the last 25

years of combustion modelling allied to a forward

projection by Cathonnet [7]. Volume 35 in a series on

comprehensive chemical kinetics entitled ‘Low Tempera-

ture Combustion and Autoignition’ [8] contains many

interesting chapters.1

Other reviews have a slightly different focus such as

Westbrook [9] on the chemical kinetics of hydrocarbon

ignition in practical combustion systems, Ranzi et al. [10] on

lumping procedures in detailed chemical kinetic modelling

of gasification, pyrolysis, partial oxidation as well as

combustion of hydrocarbon mixes, Lindstedt [11] on

modelling complexities of hydrocarbon flames, Richter

and Howard [12] and Frenklach [13] on the formation of

polycyclic aromatic hydrocarbons and soot, Williams [14]

on detonation chemistry, and, Battin-LeClerc [15] on the

development of kinetic models for the combustion of

unsaturated hydrocarbons.

More specific topics which are of importance in

combustion modelling include a plenary lecture on theory

and modelling in combustion chemistry by Miller [16], an

insightful review of unimolecular falloff by Kiefer [17],

Gardiner’s book on gas-phase combustion chemistry [18]

and a handbook of chemical reactions in shock waves [19].

This review will consider post-1994 work and will focus

on the modelling of hydrocarbon oxidation in the gas-phase

by detailed chemical kinetics and those experiments which

validate them. The word detailed is used for those models

which attempt to describe at the molecular level the

chemical changes which occur during combustion and

which is essential for trace species predictions. Of course the

actual number of elementary reactions required to outline a

particular reaction mechanism can be the subject of debate

with many authors using truncated or skeletal mechanisms

which neglect some intermediates completely or which do

not differentiate between various quantum states of one

compound.

As the number of steps and species required to describe a

particular oxidation process increases, the computational

burden can become too great and methods of simplification

are needed; this is an active area of research, see for

example, work on reduced kinetic schemes based on

intrinsic low-dimensional manifolds by Maas and co-

workers [20], on reaction rate analysis by Sung et al. [21]

and on computational singular perturbation by Massis et al.

[22] and by Valorani and Goussis [23]. Once this reduction

has taken place then simulation of a complex combustion

device can proceed [24].

1.1. Reaction mechanism design

The design of a reaction mechanism is still a black art with

the majority being constructed on an ad hoc basis relying

heavily on intuition, rules of thumb, etc. and building on

previous sub-mechanisms. The computer-aided design

1 Although dated 1997 it appears to have been mainly completed

in 1995.

J.M. Simmie / Progress in Energy and Combustion Science 29 (2003) 599–634600

approach (or logical programming as Tomlin et al. [25] refer

to it in their overview of the mathematical tools for the

construction, investigation and reduction of combustion

mechanisms) has been applied by several groups, see for

example, work by Ranzi et al. [26], Come et al. [27,28],

Nehse et al. [29] but has not had the impact or the success that

might have been expected. An alternative, more optimistic

view of progress is given by Green et al. [30].

The approach used to produce the well-known Gas

Research Institute methane mechanism was outlined by

Frenklach et al. [31] and bears repeating here, lightly

paraphrased, because it encapsulates what is probably best

practice:

1. Assemble a reaction model consisting of a complete set

of elementary reactions.

2. Assign values to their rate constants from the literature or

by judicious estimation; treating temperature and

pressure dependences in a proper and consistent manner.

Evaluate error limits and thermodata required for the

calculation of equilibrium reverse rate constants.

3. Carry out and/or find in the literature reliable exper-

iments that depend on some or all of the rate and

transport parameters in the model.

4. Use a computer application to solve the reaction

mechanism kinetics and any transport equations, com-

puting values of the observables for these ‘target’

experiments. Also apply sensitivity analysis to determine

how the model rate constants affect the final result.

5. Choose experimental targets sensitive to a representative

cross-section of the rate parameters. Also select those

parameters making the largest impacts on a given target;

these are then the potential optimisation parameters.

1.2. Modelling applications

There are a number of different computer applications

[32–36] available to the chemical kinetic modelling

community with Chemkin-III [37] probably the dominant

one, perhaps because the Chemkin input data format [38] is,

de facto, an evolving standard for describing the reactions,

the rate parameters and the thermodynamic and transport

properties of species. The transferability of this data is

crucial to advancing the field but the current formats are not

helpful relying as they do on cumbersome notation such as

inline formulas for describing species; for example,

although pC3H4 and aC3H4 are sufficient to distinguish

between propyne (H3C–CxCH) and allene (H2CyCyCH2)

more complicated species are virtually impossible to specify

unambiguously in this fashion. Some hope is held out for

XML, a metalanguage for describing markup languages

[39], but to date, there are no working models from which

one can judge the utility of such a concept. Although there is

interesting ongoing work, such as OpenChem Workbench

[40] which, if brought to fruition, might knit together kinetic

modelling and computational chemistry in a seamless whole

and circumvent many of the problems encountered today.

Whilst the primary focus here is on the mechanism and

rates of reactions (Baulch [41] discusses the data needs for

combustion modelling, whilst Sumathi and Green [42]

review progress in obtaining fast and accurate estimates of

rate constants) it must not be forgotten that the thermodyn-

amic [43] and transport data of species are probably equally

important and in some cases more important. Thus, an

adiabatic flame temperature is mainly sensitive to the

enthalpies of formation of key species such as OzH [44].

Numerical concerns have been addressed by Schwer et al.

[45] who have studied the effect of re-writing legacy Fortran

coding, as exemplified by Chemkin-II, and shown a factor of

ten reduction in computation time for an n-heptane

calculation whilst Manca et al. [46] consider new numerical

integration methods, used to solve the coupled differential

and algebraic equations in order to determine species

concentrations as a function of time. While Song et al. [47]

explore the interaction between the structure, that is the

number of reactions and species, of a chemical kinetic

model and the parameter range over which it is applied, that

is the concentration ranges, and although they discuss the

pyrolysis of methane/ethane mixtures, their findings apply

equally to combustion.

1.3. Environments

Since combustion experiments can be carried out in

many different environments2 (depending on the geometry

of the equipment, the pressure and temperature ranges

spanned, etc.; see, for example, Ref. [52]) the modelling

application must not only model the chemistry but also the

environment. Thus, Chemkin-III provides modelling of

shock tubes, premixed flames, diffusion flames, partially and

perfectly stirred reactors, internal combustion engines,

stagnation flow, rotating-disk reactors, cylindrical or planar

channel flow, and well mixed plasma reactors. In most cases

these environments are treated as ideal, with symmetry and

other considerations being used to minimise complexity;

Roesler [53], is one of the few to address these issues, when

he explored the performance of a laminar, non-plug-flow

reactor in methane–O2 and CO/H2–O2 in a 2D-modelling

and experimental study. Whilst Gokulakrishnan et al. [54]

have considered the kinetic difficulties posed by the mixing

of reactants before entry to a variable pressure flow reactor

and shown that the ‘time shifting’ technique that this group

normally uses in comparing experiment and simulation can

be problematic.

More complex environments may also be modelled via

so-called reactor networks, that is, combinations of plug

and/or perfectly stirred reactors; thus a complex environment

2 Note that gas-phase mechanisms may well be of use in

describing the oxidation of hydrocarbons [48–50] in supercritical

water [51].

J.M. Simmie / Progress in Energy and Combustion Science 29 (2003) 599–634 601

can be broken down, via three-dimensional (3D) compu-

tational fluid dynamics (CFD) calculations, into simplified

flow models and detailed chemical kinetics employed [55,

56]. This approach will be of increasing importance,

particularly for industrial applications (for a good example,

see the work by Niksa and Liu [57] who employed ,56

continuously-stirred tank reactors to estimate NOX emissions

in a coal-fired furnace), but is only an interim measure, with

the addition of detailed chemical kinetic modelling to CFD

calculations being the next big challenge for the kineticist,

although Williams [14] states that this aim still remains too

challenging.

2. Alkanes

Hydrocarbons are by far and away the best studied class

of compounds for which reliable and detailed chemical

kinetic models for combustion exist. This is not surprising

given that the bulk of automotive fuels are comprised almost

exclusively of hydrocarbons.

Detailed chemical mechanisms describing hydrocarbon

combustion chemistry are structured in a hierarchical

manner with hydrogen–oxygen–carbon monoxide chem-

istry at the base, supplemented as needed by elementary

reactions of larger chemical species and reactions of

nitrogen species if air is used as the oxidant. So, a validated

comprehensive mechanism for H2/CO/(N2) is an essential

starting point

2.1. Methane

The lower alkanes are probably the most extensively

studied and hence the best understood chemical kinetic

models with methane in particular being the object of a

sustained campaign culminating in the Gas Research

Institute study [58]. The mechanism was developed and

published in a number of electronic versions [59] but ceased

as of February 2000 and was primarily constructed to

describe the ignition of methane and natural gas3 including

flame propagation as well.

The last version, GRI-Mech 3.0, consisted of 325

elementary chemical reactions and associated rate coeffi-

cient expressions and thermochemical parameters for the 53

species optimized (within a set of constraints) to perform

over the ranges 1000–2500 K, 1.0–1000 kPa, and equival-

ence ratios from 0.1 to 5 for premixed systems. Some

aspects of natural gas combustion chemistry such as soot

formation are not described by GRI-Mech 3.0 and although

species such as methanol and acetylene are present in the

mechanism, the GRI model cannot be used, for example, to

describe the burning of pure methanol. In spite of these

shortcomings the methodology employed was of the highest

quality with specific targets in mind and attempts being

made to model widely differing experiments. The ready

availability of the complete dataset, including that of the

validating experiments (too numerous to quote here), in an

electronic form over the Internet was another very positive

feature and undoubtedly contributed to its widespread use. It

is most unfortunate that such a useful collaborative venture,

which began accidentally, has been extinguished.

An experimental and modelling study of methane (and

ethane) oxidation at atmospheric pressure between 773 and

1573 K was carried out by Barbe et al. [60] in both

isothermal perfectly stirred and tubular flow reactors. They

determined stable species profiles and matched these against

a mechanism of 835 reactions and 42 species.

Although GRI-Mech did include a very comprehensive

set of experiments (targets) including shock-tube ignition

delay and species profiles measurements, laminar flame

speed and species profiles, and, flow and stirred reactor data

it did not consider the work of Musick et al. [61] who

measured species profiles of Hz, H2, CzH3, HOz, H2O, C2H2,

CO, CO2, C3H3, C3H4, C4H2 and CH4 by molecular beam

mass spectrometry in five methane–oxygen–argon low

pressure flames. In this strangely neglected paper they

compared their results with eight then-current mechanisms

for methane, Table 1, concluding that none of them gave a

satisfactory account of experiment. Finally they proposed a

new mechanism, MMSK, which performed better but still

needed an improved understanding of C3 and C4 chemistry.

Another purely electronic detailed reaction mechanism

for methane and natural gas combustion due to Konnov [69]

also deals with C2 and C3 hydrocarbons and their

derivatives, n-H–O chemistry and NOx formation in flames.

The mechanism comprises some 1200 reactions and 127

species and is extensively validated against a large dataset of

experiments including species profiles and ignition delay

times in shock waves, laminar flame species profiles,

laminar flame speeds, and, temperature and stable species

concentration profiles in flow reactors-although the bulk of

the validating experiments are focused on H2, CO, N2O,

Table 1

Characteristics of methane mechanisms [61]

Reactions Reversible Species Remarks Reference

82 All C1–C2 No third body [62]

168 All C1–C2 Third body;

fall-off

[59]

67 Some C1–C2 No third body [63]

141 All C1–C4 Third body;

fall-off

[64]


89 Some C1–C2 No third body;

fall-off

[66]


138 Some C1–C4 Third body [68]3 Real natural gas is a mixture of highly variable composition and

hence quite difficult to model.


NO2 and NH3 kinetics rather than on the hydrocarbons CH4,

C2H6, and C3H8.

Methane pyrolysis and oxidation was studied by Hidaka

et al. [70] behind reflected shock waves in the temperature

range 1350–2400 K at pressures of 162–446 kPa. Methane

decay in both the pyrolysis and oxidation reactions was

measured by using time-resolved infrared laser absorption at

3.39 mm, CO2 by IR emission at 4.24 mm and the product

yields were also studied using the single-pulse technique.

The pyrolysis and oxidation of methane were modelled

using a kinetic reaction mechanism, with 157 reaction steps

and 48 species, including the most recent mechanism for

formaldehyde, ketene, acetylene, ethylene, and ethane

oxidations.

The ignition of methane–oxygen mixtures of equival-

ence ratio 0.4–1.0, highly dilute in argon, was determined

by Jee et al. [71] at 1520–1940 K and in reflected shock

waves with initial pressures of 2.7 kPa; the results were

modelled with GRI-Mech 1.2 with which they were in

satisfactory agreement.

Crunelle et al. [72,73] studied premixed laminar

methane/O2/Ar low pressure flat flames at three equival-

ence ratios (0.69, 1 and 1.18) measuring a large number of

species profiles by molecular beam mass spectrometry as

a function of height above the burner. They compared

their results with the predictions of a natural gas

mechanism by Tan et al. [74] and with GRI-Mech 2.11;

the latter gives good agreement with experiment, except

for C3 species for which it is not parameterised. The Tan

mechanism covers a wider range of species, up to C6, and

was updated to give a good account of the results although

the rates of formation and consumption of formaldehyde

are still discrepant.

Petersen et al. [75] conducted an analytical study to

supplement extreme shock-tube measurements of CH4/O2

ignition [76] at elevated pressures (4–26 MPa), high

dilution (fuel plus oxidizer #30%), intermediate tempera-

tures (1040–1500 K), and equivalence ratios as high as 6. A

38-species, 190-reactions model (RAMEC), based on GRI-

Mech 1.2, was developed using additional reactions that are

important in methane oxidation at lower temperatures. The

detailed-model calculations agree well with the measured

ignition delay times and reproduce the accelerated ignition

trends seen in the data at higher pressures and lower

temperatures. Although the expanded mechanism provides a

large improvement relative to the original model over most

of the conditions of this study, further improvement is still

required at the highest CH4 concentrations and lowest

temperatures. Sensitivity and species flux analyses were

used to identify the primary reactions and kinetic pathways

for the conditions studied. In general, reactions involving

HOz

2, CH3Oz

2, and H2O2 have increased importance at the

conditions of this work relative to previous studies at lower

pressures and higher temperatures. At a temperature of

1400 K and pressure of 10 MPa, the primary ignition

promoters are:

CzH3 þ O2 ¼ O þ CH3Oz V CH2O þ Hz

HOz

2 þ CzH3 ¼ OzH þ CH3Oz

Methyl recombination to ethane is a primary termination

reaction and is the major sink for CzH3 radicals. At 1100 K

and 10 MPa, the dominant chain-branching reactions

become:

CH3Oz

2 þ CzH3 ¼ CH3Oz þ CH3Oz

H2O2 þ M ¼ OzH þ OzH þ M

These two reactions enhance the formation of Hz and OzH

radicals, explaining the accelerated ignition delay time

characteristics at lower temperatures.

Mertens [77] has studied the reaction kinetics of CzHp

(a useful diagnostic for determining ignition) in shock-

heated CH4/O2/Ar and C2H2/O2/Ar mixtures, at 2880–

3030 K and 100–152 kPa, by comparing emission traces

at 431 nm against simulated CzHp concentrations based on

a GRI-Mech 2.11 model. He concludes that HCxCz þ

O2 ! CzHp þ CO2 proceeds at higher rates than pre-

viously thought and recommends a rate constant of

9:0 £ 1012 expð21780=TÞ cm3 mol21 s21. Walsh and co-

workers [78] have extended measurements on a lifted

axisymmetric laminar nitrogen–diluted methane diffusion

flame to measure CzH, CzHp and OzHp and to model their

two-dimensional (2D) results quite successfully (except

for peak concentrations of CzHp) with both GRI-Mech

2.11 and a 26-species C2 mechanism due to Smooke et al.

[79], after adding in additional reactions to account for the

production and destruction of excited state species, CzHp

and OzHp, which are absent from most mechanistic

schemes.

A comprehensive methane oxidation mechanism [80],

due to Hughes et al. [81], has appeared which also deals

with the oxidation kinetics of hydrogen, carbon mon-

oxide, ethane and ethene in flames and homogeneous

ignition systems. This Leeds mechanism (version 1.4)

consists of 351 irreversible reactions of 37 species, built

with an overall philosophy akin to that of GRI-Mech,

and using much the same set of experiments on laminar

flame speeds [82–88], ignition delay measurements

[89–92] and species profiles in laminar flames [93],

except that the authors argue that their approach is less

restrictive and can be used unaltered to construct more

elaborate kinetic mechanisms. This latter point is almost

certainly untrue, since the validating experiments never

illuminate equally all of the reactions in a postulated

mechanism; subsequent studies on higher fuels may well

throw light on aspects of the methane model which then

needs to be re-evaluated.

The overall performance of the Leeds model is similar to

that of GRI-Mech and other earlier models although many

of the most important reactions differ significantly;


the conclusion that the authors draw is noteworthy and

disturbing—the chemistry of oxidation of simple fuels

such as CO, CH4 and C2H6 is still not yet well characterised

at the elementary level. Thus, for example, the Leeds and

RAMEC [75] mechanisms predict comparable ignition

delays, Fig. 1, but use very different rate expressions4 for

the reactions:

CH4 þ O2 ! CzH3 þ HOz

2

CzH3 þ H2O2 ! CH4 þ HOz

2

H2CO þ O2 ! HCzO þ HOz

2

McEnally et al. [95] studied a non-sooting methane/air

coflowing non-premixed flame carrying out 2D mapping

of gas temperature, major (CH4, O2, CO, CO2, H2) and

minor (C2H2, C6H6, H2CCO) species concentrations with

both probes (thermocouples and mass spectrometry) and

optical diagnostics (Rayleigh and Raman scattering).

The detailed chemical kinetic model of 140 reactions

and 39 species, adapted from earlier work [79], is quite

good at predicting both major and minor species to within

experimental error.

Gurentsov et al. [96] have modelled the ignition delay

times that they obtained for pure methane and methane/

water/carbon monoxide/hydrogen mixtures using a kinetic

scheme for methane–air combustion due to Dautov and

Starik [97] with only moderate success.

In an interesting paper Turanyi and colleagues [44]

analyse the effect of uncertainties in kinetic and thermo-

dynamic data on the simulation of premixed laminar

methane–air flames using the Leeds methane oxidation

mechanism. They conclude that accurate enthalpies of

formation for the species OzH5, CH2(S), CzH2OH, HCzCyO

and CzH2CHO are required as well as refined values for the

rates of the reactions:

Hz þ O2 ! OzH þ O

Hz þ O2 þ M ! HOz

2 þ M

OzH þ CO ! Hz þ CO2

Hz þ CzH3 þ M ! CH4 þ M

OzH þ CzH3 ! CH2ðSÞ þ H2O

OzH þ HCxCH ! HCxCz þ H2O

CzH þ HCxCH ! HCxCz þ CH2

and they remind us that simulations should be accompanied

by an uncertainty analysis.

Fig. 1. Leeds (- - ) vs. RAMEC (—) mechanisms; data from Fig. 5 of Ref. [75].

4 Comparisons can be difficult sometimes because either forward,

kF; or reverse, kR; rate constants may be quoted; an excellent tool,

GasEq, for transforming kF , kR and other calculations, is

available [94].

5 Ruscic et al. [98] and Herbon et al. [99] have recently revised

DHF(OzH, 298 K) downwards to 37.3 ^ 0.67 kJ/mol.


Rozenchan et al. [100] have measured stretch-free

laminar burning velocities for methane–air flames up to

2 MPa and methane–oxygen–helium flames up to 6 MPa.

Simulation with GRI-Mech 3.0 shows good agreement with

these and other recent experiments [101–103] for pressures

up to 2 MPa but substantial disagreement above this

pressure which is unsurprising given that the mechanism

was not calibrated over this extended pressure range.

Lamoureux et al. [104] have measured ignition delays

for methane, ethane and propane from 1200 to 2700 K, 0.1–

1.8 MPa, equivalence ratios of 0.5–2 and in high dilution in

argon. As well as presenting a useful summary of all

previous work they also model their data with three detailed

mechanisms. The first, due to Tan et al. [74] contains 78

species and 450 reactions and is essentially a natural gas

mechanism as is the second, GRI-Mech 3.0, whilst the third

is a methane oxidation mechanism due to Frenklach and

Bornside [105]. Shortcomings in all three are noted.

Davidenko et al. [106] surveyed a number of methane

mechanisms, GRI-Mech 1.2 [107], GRI-Mech 3.0 [58],

Princeton [108], Leeds 1.5 [81], LCSR [109] and LLNL

[110], on their way to producing skeletal models which they

could employ in multi-dimensional simulations of complex

reacting flows; they were surprised by the lack of agreement

between the models and concluded that the LCSR

mechanism performed best—but on the basis of a quite

limited comparison with shock-tube experiments [111,112].

A comprehensive re-evaluation of all extant methane

mechanisms would be welcome if exceedingly time-

consuming; Rolland [113] is currently developing Visual

Basic software tools for the automatic comparison of

Chemkin-formatted mechanisms. Tables 2 and 3 show

the inter-relationships as regards species and reactions for

a number of methane mechanisms including Konnov

[69], NUIG [114], BGD [115], GRI-Mech 3.0 [58] and

Leeds [81].

The figures in brackets in Table 2 are the total number of

species for the considered mechanism. The intersection of

row and column shows the number of species in common

between two mechanisms. Thus there are, for example, 48

species in common between the GRI-Mech and Konnov

mechanisms, which means that around 91% of GRI-Mech’s

species are present in the Konnov mechanism.

In Table 3, the figures in brackets are the total number of

individual reactions in the considered mechanism. Two

numbers are present at the intersection; the first corresponds

to the number of identical (exactly the same reactants,

products and Arrhenius parameters) reactions between the

two mechanisms. The second is the number of otherwise

identical reactions but which are numerically different, in a

non-trivial sense. For instance, the Leeds and Konnov

mechanisms share 142 similar reactions, 76 of which are

identical but the remaining 66 differ numerically.

2.1.1. Mixtures

While studying the combustion chemistry of a pure

compound is the best starting point for any investigation,

studies of mixtures are often illuminating and are, of course,

economically important [116]. There is a large body of work

in this area and here we present only a select few.

Spadaccini and Colket [117] determined ignition delay

times for mixtures of methane with ethane, propane and

butane and for typical natural gas at 1300–2000 K,

pressures of 0.3–1.5 MPa and equivalence ratios of 0.45–

1.25 in a comprehensive paper which also summarises much

previous work. They adopted a mechanism due to Frenklach

et al. [31], adapting it slightly and finding generally

consistent agreement with experiment.

Tan et al. [118] studied the kinetics of oxidation of

methane/ethane blends in a jet-stirred reactor at 850–

1240 K and 100–1000 kPa, whilst Yang et al. [119] carried

out a combined experimental and modelling study of

methane and methane mixtures with ethane and propane.

Ignition delays in shock-heated CH4 þ O2 mixtures, with

and without ignition-promoting additives, account for most

of the available validation data. Additional reflected shock

wave ignition experiments were done to explore possible

reasons for the mismatches and to study the promotion of

CH4 ignition by small amounts of C2H6 and C3H8 additives.

Empirical correlations were derived that describe ignition

delays in CH4 þ C2H6 þ C3H8 þ O2.

Even more complex mixtures such as natural gas,

kerosene and gas oil are discussed by Dagaut [120] who

has developed simple mixtures in an attempt to simulate the

actual fuel. Thus he shows that an appropriate methane–

ethane–propane blend is a good representation of natural

gas, whilst Violi et al. [121] attempt to model JP-8 fuel with

a blend of m-xylene, iso-octane, methylcyclohexane,

dodecane, tetradecane and tetralin.

Future work will increasingly focus on multi-dimen-

sional modelling as exemplified by the recent study of

Table 2

Species in common

Konnov (127)

41 NUIG (81)

60 34 BGD (77)

48 32 48 GRI-Mech

3 (53)

34 29 31 30 Leeds (37)

Table 3

Reactions in common

Konnov (1207)

140; 196 BGD (484)

29; 121 42; 79 NUIG (356)

56; 160 75; 150 35; 68 GRI-Mech

3 (325)

76; 66 48; 80 15; 67 21; 90 Leeds

(175)


Agarwal and Assanis [122] who reported on the autoignition

of natural gas injected into a combustion bomb at pressures

and temperatures typical of top–dead–center conditions in

compression ignition engines. This study combined a

detailed chemical kinetic mechanism, consisting of 22

species and 104 elementary reactions, with a multi-

dimensional reactive flow code. The effect of natural gas

composition, ambient density and temperature on the

ignition process was studied by performing calculations

for three different blends of natural gas on a 3D

computational grid. The predictions of ignition delay

compared very well with measurements in a combustion

bomb. It was established that a particular mass of fuel

burned is a much better criterion to define the ignition delay

period than a specified pressure rise. The effect of additives

like ethane and hydrogen peroxide in increasing the fuel

consumption rate as well as the influence of physical

parameters like fuel injection rate and intake temperature

was studied. It was thus shown that apart from accurate

predictions of ignition delay the coupling between multi-

dimensional flow and multi-step chemistry is essential to

reveal detailed features of the ignition process.

2.2. Ethane

Ethane oxidation has been studied by Hunter et al. [123]

in the intermediate temperature regime under lean con-

ditions using a flow reactor. Species profiles have been

obtained for H2, CO, CO2, H2CO, CH4, C2H4, C2H6, C2H4O

(ethylene oxide or oxirane), and CH3CHO at pressures of

300, 600, and 1000 kPa for temperatures ranging from 915

to 966 K using a constant equivalence ratio of 0.2 (in air).

To model this data a detailed chemical kinetic model for

ethane oxidation was developed and expanded, from a GRI-

Mech 1.1 basis, to include reactions pertinent to the lower

temperatures and elevated pressures. The expanded mech-

anism consists of 277 elementary reactions and contains 47

species. By adjusting the rate coefficients of two key

reactions

CH3Oz

2 þ Cz

3 ! CH3Oz þ CH3Oz

CH3CH3 þ HOz

2 ! CH3CzH2 þ H2O2

the model was brought into agreement with experiment at

600 kPa; however, the model indicates a larger pressure

sensitivity than was measured experimentally. Note too that

the model takes measured values obtained at the first

sampling point as its starting point and not as is more

commonly done the initial input streams of reactants; this

was done to obviate the mixing problem. Results indicate

that HOz

2 is of primary importance in the regime studied

controlling the formation of many of the observed

intermediates including the aldehydes and ethylene oxide.

The results also point to the importance of continued

investigation of the reactions of HOz

2 with C2H6, CH3CzH2,

and C2H4 to further the understanding of ethane oxidation in

the intermediate temperature regime. The expanded mech-

anism has also been tested against shock-tube ignition delay

[91,124] and laminar flame speed data and was found to be

in good agreement with both the original GRI-Mech and the

experimental data for both methane and ethane.

Pyrolysis and oxidation of ethane were studied behind

reflected shock waves by Hidaka and co-workers [125] in

the temperature range 950–1900 K and at pressures of 120–

400 kPa using the same techniques as those they used for

methane [70]. The present and previously reported shock-

tube data were reproduced using this mechanism and

comparisons drawn with GRI-Mech 1.2 and one due to

Dagaut et al. [126]. The rate constants of the reactions

C2H6 ! CzH3 þ CzH3

C2H6 þ Hz ! CH3CzH2 þ H2

CH2CH2 þ Hz ! CH3CzH2

CH3CzH2 þ Hz ! H2CyCH2 þ H2

CH3CzH2 þ O2 ! H2CyCH2 þ HOz

2

are discussed in detail as they are important in predicting the

previously reported and the present data.

An experimental and numerical investigation on ethane–

air two-stage combustion in a counterflow burner where the

fuel stream, which is partially premixed with air for

equivalence ratios from 1.6 to 3.0, flows against a pure air

stream was reported by Waly et al. [127]. The two-stage

ethane combustion exhibits a green, fuel-rich, premixed

flame and a blue diffusion flame. Flame structures, including

concentration profiles of stable intermediate species such as

C2H4, C2H2 and CH4, are measured by gas chromatography

and are calculated by numerical integrations of the

conservation equations employing an updated elementary

chemical-kinetic data base. The implications of the results

from these experiments and numerical predictions are

summarized, the flame chemistry of ethane two-stage

combustion at different degrees of premixing (or equival-

ence ratio) is discussed, and the relationship between NOx

formation and the degree of premixing is established.

Ikeda and Mackie [128] have modelled ignition delays in

shock-heated ethane–oxygen mixes with 0:68 # f # 1:7

between 1155 and 1500 K and at pressures of 1–1.5 MPa

with GRI-Mech 3.0; they have shown that some additional

reactions are necessary, including:

C2H6 þ O2 ¼ CH3CzH2 þ HOz

2

C2H6 þ HOz

2 ¼ CH3CzH2 þ H2O2

CH3CzH2 þ HOz

2 ¼ CH3CH2Oz þ OzH

CH3CzH2 þ HOz

2 ¼ CH2CH2 þ H2O2

Extremely high-pressure oxidation measurements have

been made by Tranter et al. [129] in a remarkable single


pulse shock tube at 1050–1450 K and at 34 and 61.3 MPa,

and latterly at 4 MPa [130]; the dwell times range from 1.12

to 1.68 ms [131]. Such conditions present a severe test of

reaction mechanisms as they lie well outside the typical

range of most studies. The authors tested three existing

mechanisms, GRI-Mech [58], Marinov et al. [132], and,

Pope and Miller [133] and found that this last mechanism

(developed to explore the formation of benzene in low-

pressure, ethyne, ethene and propene laminar flat flames)

performs best at simulating the high-pressure oxidation

results, although it is not quite as good as the other two at

accounting for pyrolytic reactions.

The low temperature, 500–900 K, oxidation of ethane

(and propane) has been tackled by Naik et al. [134] who

applied their model to rate measurements for CH3CzH2 þ

O2 by Slagle et al. [135], to ethylene yields by Kaiser et al.

[136] and to ignition data of Knox and Norrish [137] and

Dechaux and Delfosse [138]. They suggest that the

elimination reaction CH3CH2OOz ! CH2CH2 þ HOz

2 pro-

ceeds much faster than the isomerisation CH3CH2OOz !

CzH2CH2OOH; therefore that the branching process CzH2-

CH2OOH þ O2 will be impeded and consequently ethane

will not show negative temperature coefficient (NTC)

behaviour.

2.3. Propane

A study of detailed chemical kinetics in coflow and

counterflow propane (and methane) diffusion flames was

presented by Leung and Lindstedt [139] using systematic

reaction path flux and sensitivity analyses to determine the

crucial reaction paths in propane and methane diffusion

flames. The formation of benzene and intermediate hydro-

carbons via C3 and C4 species has been given particular

attention and the relative importance of reaction channels

has been assessed. The developed mechanism considers

singlet and triplet CH2, isomers of C3H4, C3H5, C4H3, C4H5

and C4H6 for a total of 87 species and 451 reactions.

Computational results show that benzene in methane–air

diffusion flames is formed mainly via reactions involving

propargyl radicals and that reaction paths via C4 species are

insignificant. It is also shown that uncertainties in

thermodynamic data may significantly influence predictions

and that the reaction of acetylene with the hydroxyl radical

to produce ketene may be an important consumption path

for acetylene in diffusion flames. Quantitative agreement

has been achieved between computational results and

experimental measurements of major and minor species

profiles, including benzene, in methane–air and propane–

air flames. It is also shown that the mechanism correctly

predicts laminar burning velocities for stoichiometric

propane (and methane) flames.

A pressure-dependent kinetic mechanism for propane

oxidation was developed by Koert et al. [140] to match high-

pressure flow reactor experiments from 1 to 1.5 MPa, 650–

800 K and dwell time of 198 ms for a 1:1 propane:O2 mix in

nitrogen. The data show a dramatic NTC region between

720 and 780 K where the rate of overall reaction decreases

with increasing temperature. The model gives a satisfactory

fit to the bulk of the data but formaldehyde, acetaldehyde,

ethylene and CO2 concentrations are out by factors of 3 and

acrolein is 20 times less abundant than the model

predictions.

Qin et al. [141] undertook a computer modelling study to

discover whether optimizing the rate parameters of a 258-

reaction C3 combustion chemistry mechanism that was

added to a previously optimized 205-reaction C,3 mechan-

ism would provide satisfactory accounting for C3 flame

speed and ignition data. It was found in sensitivity studies

that the coupling between the C3 and the C,3 chemistry was

much stronger than anticipated. No set of C3 rate parameters

could account for the C3 combustion data as long as the

previously optimized (against C,3 optimisation targets

only) C,3 rate parameters remained fixed. A reasonable

match to the C3 targets could be obtained without degrading

the match between experiment and calculation for the C,3

optimisation targets, by reoptimizing six of the previously

optimized and three additional C,3 rate parameters. In

essence then, this study shows that optimising a reaction

sub-mechanism does not guarantee that further optimisation

will not be required as the mechanism is expanded—put

simply, studying larger fuels can still teach you something

new about smaller fuels.

Cadman et al. [142] have determined autoignition times

of up to 6 ms duration in quite concentrated mainly lean

propane-air mixtures in a monatomic bath gas, from 850 to

1280 K and at pressures between 0.5 and 4 MPa; the rather

long dwell times were obtained by tailoring of the driver

gas. They compared their delay times with predictions based

on mechanisms originally by Jachimowski [143], Dagaut

et al. [144] and Voisin [109], none of which could account in

a satisfactory manner for the experiments even when these

mechanisms were supplemented with additional reactions in

an attempt to enhance coverage of the chemistry below

1110 K. Propane autoignition times at 4 MPa and 750–

1050 K measured by Gallagher [145] in a rapid compression

machine are substantially longer than those observed by

Cadman et al. and are in better agreement with the

predictions of the Voisin mechanism [109].

Kim and Shin [146] investigated the ignition of propane

behind reflected shock waves in the temperature range of

1350–1800 K and the rather narrow pressure range of 75–

157 kPa. The ignition delay time, t; was measured from the

increase of pressure and OzH emission and they present a

relationship between t and the concentrations of propane

and oxygen, but not the third component argon, in the form

of the usual mass-action expression with an Arrhenius

temperature dependence, t ¼ A½C3H8�a½O2�

bexpðu=TÞ:

Numerical calculations were also performed to elucidate

the important steps in the reaction scheme using various

mechanisms due to Qin [147], Sung et al. [148], Glassman

[149], Konnov [150] and GRI-Mech 3.0 [58]. The measured


ignition delay times were in best agreement with those

calculated from the mechanism of Sung et al. [148], which

was developed from temperature and species measurements

in propane/nitrogen diffusion flames.

Davidson et al. [151] have obtained OzH concentration

time histories during the ignition of stoichiometric

propane/oxygen mixtures highly dilute in argon

($99.1%) between 1500 and 1690 K at an average

reflected shock pressure of 220 kPa. These high-quality

measurements (also obtained for butane, n-heptane and n-

decane) show an initial rapid rise in [OzH] to a constant

value and then a later rise to an essentially constant post-

ignition value; the authors ascribe this behaviour to an

initial rapid growth of OzH, formed in branching reactions,

this phase is then succeeded by a period during which

propane or its decomposition products keeps the popu-

lation in check followed by a third phase during which

OzH removal is no longer so important and therefore [OzH]

increases unchecked. The performance of three propane

models, due to Smith et al. [58], Qin et al. [141] and

Laskin et al. [152], was compared with the data and

although all three give a reasonable fit to the ignition time

none of them give a satisfactory account of the complete

concentration–time history. Sensitivity analysis shows,

unsurprisingly, very large sensitivity towards Hz þ O2 !

O þ OzH but other reactions do figure, with fuel

decomposition featuring strongly initially.

The reduction of nitric oxide by propane in simulated

conditions of the reburning zone has been studied in a fused

silica jet-stirred reactor operating at 100 kPa from 1150 to

1400 K by Dagaut et al. [153]. Some work on the neat

oxidation of propane ðf ¼ 1:25Þ highly dilute in nitrogen

(2930 ppm of propane) at 100 kPa from 1000 to 1350 K is

also reported. A detailed chemical kinetic modeling of these

experiments was performed using an updated and improved

kinetic scheme of 892 reversible reactions and 113 species

with overall reasonable agreement.

The oxidation and combustion of propane has been

modelled for low temperatures, 500–800 K, and pressures

of 200 Pa to 1.5 MPa by Barckholtz et al. [154]; a copy of

GRI-Mech version 2.11 was enhanced to 3078 reactions of

216 species by including C4 chemistry and compared to the

HOz

2 yield data of DeSain et al. [155] for the reaction

CH3CH2CzH2 þ O2, the flow tube data of Koert et al. [156]

and the static reactor experiments of Wilk et al. [157] with

quite good agreement.

The low temperature, 500–900 K, oxidation of propane

(and ethane) has been tackled by Naik et al. [134] who

applied their extended ethane model to yield measurements

of HOz

2 from CH3CzH2 þ O2 by DeSain et al. [155], to

ignition times by Wilk et al. [157], and, to propane

consumption data of Koert et al. [156] with some success.

They conclude that for propane the isomerisation path is

faster than concerted elimination and produces NTC

behaviour, in contrast to their conclusions for ethane.

2.4. Butanes

The complexity of isomerisation arises now with two

different butanes, of molecular formula C4H10, the straight-

chain version n-butane and the branched-chain iso-butane;

for pentadecane, C15H32, there will be 4347 isomers.

An experimental investigation by Wilk et al. [158] has

examined the transition in the oxidation chemistry of n-

butane across the region of NTC from low to intermediate

temperatures. The experimental results, obtained in a static

cylindrical Pyrex reactor at 73 kPa for a fuel-rich mixture in

nitrogen, indicated a region of NTC between approximately

640 and 695 K and a shift in the nature of the reaction

intermediates and products across this region. On the basis

of these experimental results and earlier work by Slagle et al.

[135] for the reaction Rz þ O2 O ROz

2, a new mechanism is

presented for n-butane to describe the observed phenomena.

At low temperatures the major reaction path of butylperoxy

is isomerization, followed by O2 addition, further isomer-

ization, and decomposition to mainly carbonyls and OzH

radicals. At intermediate temperatures, the major reaction of

butylperoxy is isomerization followed by decomposition to

butenes and HOz

2 and to epoxides and OzH. The mechanism

is consistent with these and other experimental results and

predicts the NTC and the shift in product distribution with

temperature.

An n-butane oxidation mechanism from the Nancy group

[159], with 778 reactions involving 164 species, was

validated by modelling the normal-butane oxidation at low

temperature between 554 and 737 K, in the NTC region, and

at a higher temperature of 937 K. The system yielded

satisfactory agreement between the computed and the

experimental values for the macroscopic data, induction

periods and conversions, and also for the product

distribution.

A mechanism focusing on the formation of aromatics

and polycyclic aromatics in a laminar premixed n-butane–

oxygen–argon flame ðf ¼ 2:6Þ has been developed by

Marinov et al. [132]; the atmospheric-pressure flame was

sampled for a number of low molecular weight species,

aliphatics, aromatics and 2–5-membered polycyclic aro-

matics. The model gives a reasonable account of the

concentrations of benzene, naphthalene, phenanthrene,

anthracene, toluene, ethylbenzene, styrene, o-xylene, indene

and biphenyl but a poor account of phenyl acetylene,

fluoranthene and pyrene. This bald recital of the products

found highlights the difficulties associated with giving a

proper description of what is in essence a very simple

reaction, viz. butane þO2.

A perfectly stirred reactor study by Dagaut et al. [160]

concentrated mainly on the reduction of NO by n-butane,

but also presented some results for the stoichiometric

oxidation of neat n-butane at 1050–1230 K and 100 kPa for

residence times of 160 ms. Modelling of the results for a

range of reactants, intermediates and products, whose

concentrations were measured by GC and FTIR, was


achieved with a kinetic scheme of 892 reversible reactions

and 113 species with good agreement.

Iso-butane oxidation has received very little attention; a

pyrolytic study in a static reactor [161] at 773 K included

some results in the presence of oxygen but the oxygen:iso-

butane ratio was only 1:50 and no oxidation products, other

than CO, were detected. In addition the reactor walls were

found to promote heterogeneous reactions.

An atmospheric-pressure perfectly stirred reactor study

by Dagaut et al. of NO reduction by iso-butane [162] also

obtained some results for the oxidation of iso-butane itself at

1000–1300 K, for f ¼ 0:5; 1.0 and 1.5 and with a residence

time of 160 ms. The products include CO, CO2, CH4, iso-

butene, H2CO, ethene, propene, ethane, ethyne and C4H8.6

The computed mole fractions are in generally good

agreement with experimentally determined ones except for

methane which is overestimated at temperatures .1250 K.


time histories during the ignition of a stoichiometric butane/

oxygen mixture highly dilute in argon (99.6%) between

1530 and 1760 K at an average reflected shock pressure of

210 kPa. The results parallel those of propane by the same

Stanford group (see above) and in comparing two butane

models the authors found that the one due to Marinov et al.

[132] performs slightly better than the Warth et al. [159]

mechanism.

Dagaut and Hadj Ali [163] have conducted a detailed

study of the oxidation of liquefied petroleum gas (LPG) in a

jet-stirred reactor from 950 to 1450 K at 1 bar; the LPG used

consisted of 24.8% iso-butane, 39.0% n-butane and 36.2%

propane. Kinetic modelling with a 112 species and 827

reactions mechanism gives good agreement with the

experimentally determined concentrations of fuels, inter-

mediates and partially oxidised products.

2.5. Pentanes

Autoignition of n-pentane (and 1-pentene) were studied

by Ribaucour et al. [164] in a rapid compression machine

between 600 and 900 K at pressures of 750 kPa with both

hydrocarbons showing two-stage ignition and a NTC region

and with the alkene being less reactive than the alkane.

Analysis of products found by quenching the reaction prior

to total autoignition led to a range of cyclic ethers of which

2-methyltetrahydrofuran dominated in n-pentane combus-

tion. The generalised mechanism of 975 reactions and 193

species gives a reasonable account of delay times.

Experiments in a rapid compression machine have

examined the influences of variations in pressure, tempera-

ture, and equivalence ratio on the autoignition of n-pentane

by Westbrook et al. [165]. Equivalence ratios included

values from 0.5 to 2.0, compressed gas initial temperatures

were varied between 675 and 980 K, and compressed gas

initial pressures varied from 0.8 to 2 MPa. Numerical

simulations of the same experiments were carried out using

a detailed chemical kinetic reaction mechanism. The results

are interpreted in terms of a low-temperature oxidation

mechanism involving addition of molecular oxygen to alkyl

and hydroperoxyalkyl radicals. Idealized calculations are

also reported that identify the major reaction paths at each

temperature. Results indicate that in most cases, the reactive

gases experience a two-stage autoignition. The first stage

follows a low-temperature alkylperoxy radical isomeriza-

tion pathway that is effectively quenched when the

temperature reaches a level where dissociation reactions

of alkylperoxy and hydroperoxyalkylperoxy radicals are

more rapid than the reverse addition steps. The second stage

is controlled by the onset of dissociation of hydrogen

peroxide. Results also show that in some cases, the first-

stage ignition takes place during the compression stroke in

the rapid compression machine, making the interpretation of

the experiments somewhat more complex than commonly

assumed. At the highest compression temperatures

achieved, little or no first-stage ignition is observed.

Experiments in a rapid compression machine were used

by Ribaucour et al. [166] to examine the influences of

variations in fuel molecular structure on the autoignition of

all three possible isomers of pentane; stoichiometric

mixtures of the various pentanes (2,2-dimethylpropane or

neopentane, 2-methylbutane and n-pentane) were studied at

compressed gas initial temperatures between 640 and 900 K

and at pre compression pressures of 40–53 kPa. Numerical

simulations of the same experiments were carried out using

a detailed chemical kinetic reaction mechanism.

An intermediate temperature combustion study of

neopentane by Curran et al. [167] modelled the concen-

tration profiles obtained during the addition of neopentane

to slowly reacting mixtures of H2 þ O2 þ N2 in a closed

reactor at 753 K and 67 kPa. Amongst the primary products

identified were iso-butene, 3,3-dimethyloxetane, acetone,

methane and formaldehyde.

A detailed chemical kinetic reaction mechanism (1875

reactions of 390 species) for neopentane oxidation [168]

was applied by Wang et al. to experimental measurements

taken in a flow reactor operating at a pressure of 800 kPa.

The reactor temperature ranged from 620 to 810 K, mixture

composition of 0.2% neopentane, 5.2% oxygen, and 94.6%

nitrogen, and, residence times of 200 ms. Initial simulations

identified some deficiencies in the existing model [167] and

the paper presented modifications which included upgrading

the thermodynamic parameters of alkyl radical and

alkylperoxy radical species, adding an alternative isomer-

ization reaction of hydroperoxy–neopentyl–peroxy, and a

multi-step reaction sequence for 2-methylpropan-2-yl

radical with molecular oxygen. These changes improved

the calculation for the overall reactivity and the concen-

tration profiles of the following primary products: formal-

dehyde, acetone, iso-butene, 3,3-dimethyloxetane,

methacrolein, carbon monoxide, carbon dioxide, and

6 It is unclear from the single set of results presented in Fig. 1 of

that paper to which mixture they apply.


water. Experiments indicate that neopentane shows NTC

behaviour similar to other alkanes, though it is not as

pronounced as that shown by n-pentane for example.

Modelling results indicate that this behaviour is mainly

caused by the chain propagation reactions of the hydro-

peroxyl-neopentyl radical.

The oxidation of neopentane (2,2-dimethylpropane) has

been studied by Dagaut and Cathonnet [169] experimentally

in a perfectly stirred reactor, operating at steady state, at

100, 500, and 1000 kPa, for equivalence ratios ranging from

0.25 to 2 and temperatures of 800–1230 K. The kinetics of

the oxidation of neopentane was measured by probe

sampling and off-line gas chromatography analysis of the

reacting mixtures. Concentration profiles of the reactants

(0.1–0.2 mol% neopentane/O2/N2), stable intermediates,

and products have been obtained, leading to a detailed

chemical kinetic reaction mechanism (746 reversible

reactions and 115 species). Good agreement between the

data and modelling was found. The major reaction paths for

the oxidation of neopentane have been identified through

detailed kinetic modelling.

An experimental study of the oxidation of neopentane

and iso-pentane was performed by Taconnet et al. [170] at

873 K in a perfectly or jet-stirred reactor at 84 kPa, with

dwell times of 0.4–1.2 s and for rich mixtures with f ¼ 2

and the results modelled with a generalised mechanism; this

study complements an earlier one on n-pentane by the same

group [171] also at 873 K, but at 72 kPa and times of 0.2–

0.6 s. The comparisons show that the different behaviour of

these hydrocarbons can be explained, at least in part, by the

presence of resonance-stabilized radicals.

2.6. Hexanes

Curran et al. [172] have studied the chemistry of all five

isomers of hexane by comparing a detailed kinetic model

with measurements of exhaust gases from a motored engine

operated at a compression ratio just less than that required

for autoignition. The relative ordering of the isomers as

regards critical compression ratios for ignition and the major

intermediates produced are well reproduced by the model.

Burcat and co-workers [173] measured both ignition

delay times and product distributions for methane, ethene

and propene, in preignited mixtures of n-hexane–O2–Ar

mixtures from 1020 to 1725 K and 100–700 kPa. Computer

modelling employed a 386 reaction, 61 species scheme

which was in moderate agreement with the results.

The ignition of 2-methyl-pentane, has been modelled

and compared to experiments of ignition delay time in a

shock tube by Burcat et al. [174], using 2-methyl-pentane in

mixtures with oxygen diluted with argon. The product

distribution (of methane, ethene, propene, C4HX and CO) of

the preignited mixtures were also investigated and numeri-

cal modelling of the combustion kinetics was performed.

The 2-methyl-pentane experiments were run at temperatures

of 1175 – 1770 K and pressures of 200 – 460 kPa.

The numerical modelling was performed with a large

kinetic scheme containing 430 elementary reactions, and

then reduced to a scheme containing 110 reactions—

resulting in very little difference in the predicted outcomes.

2.7. Heptanes

A 659-reaction, 109 species n-heptane combustion

mechanism of Lindstedt and Maurice [175] has been

systematically validated against data from species

profiles in counterflow diffusion flames [176] and stirred

reactors [177], and burning velocities in premixed

flames [178].

Normal heptane oxidation in a high-pressure, perfectly

stirred reactor has been investigated by Dagaut et al. [179]

from 550 to 1150 K for a stoichiometric mixture of n-

heptane and oxygen highly diluted by nitrogen at pressures

of 100–4000 kPa and residence times of 0.1–2 s. Some fifty

species were quantitatively detected and the results

discussed in terms of a generalised mechanism comprising

a low temperature region, #750 K, where the formation of

peroxy radicals is the dominant feature, and, a high-

temperature region, .750 K, where intermediate hydro-

carbons are rapidly formed and then consumed.

A perfectly stirred reactor study at 923 K by Simon et al.

[180] of n-heptane oxidation with dwell times of 0.1–0.9 s

and sub-atmospheric pressure determined quantitatively the

formation of some 16 products, and, the results modelled

with a generalised mechanism.

A rapid compression machine study of the autoignition

of n-heptane (and iso-octane) by the Lille group of Minetti

et al. [181] sampled the concentration of intermediates

during the preignition period and as well determined the

ignition delay times from 645 to 890 K.

Temperature and species mole fraction profiles have

been measured in laminar premixed n-heptane/O2/N2 (and

iso-octane/O2/N2) atmospheric pressure flames by El Bakali

and co-workers [182]. Species identification and concen-

tration measurements have been performed by GC and GC–

MS analysis. For both flames, the equivalence ratio was

equal to 1.9 and a faint yellow luminosity due to soot

particles was observed. The main objective of this work was

to provide detailed experimental data on the nature and

concentration of the intermediate species formed by

consumption of a linear or highly branched fuel molecule.

In addition to reactants and major products (CO, CO2, H2,

H2O), the mole fraction profiles of C1 (methane), C2

(ethyne, ethene, ethane), C3 (allene, propyne, propene,

propane), C4 (diacetylene, vinylacetylene, 1,2- and 1,3-

butadienes, 1-butyne, butenes), C5 (pentadienes, methyl

butenes, pentenes), C6 (hexenes, hexadienes, dimethyl

butenes, methyl pentenes) and C7 species (heptenes,

dimethyl pentenes) have been measured.

A detailed reaction mechanism of n-heptane combustion

has been elaborated by Doute et al. [183] and validated by

comparison of computed mole fraction profiles with those


measured in four premixed flames stabilized on a flat-flame

burner at 6 kPa, in a wide range of equivalence ratios (0.7–

2.0) [184]. Both stable and reactive species were measured

by molecular beam mass spectrometry. The predictions of

the model are in good agreement with the experimental

results for most stable species. The main active species Hz,

OzH and O are fairly well predicted in rich flames while

disagreements are observed in the lean and the stoichio-

metric flames. The intermediate radicals can be grouped in

three classes depending on the accuracy of the model

predictions: (i) good agreement in the whole range of

equivalence ratios, (ii) predicted mole fractions differing

from the experimental values by a constant factor in the four

flames studied, (iii) difference between computed and

measured maximum mole fractions varying from lean to

rich flames. In the discussion of the results, the observed

disagreements between the model and the experiments have

been generally interpreted in terms of experimental

inaccuracies. However, the modelling of the combustion

chemistry for heavy fuel molecules has been carried out so

far on the basis of experimental data referring only to stable

species and the problems faced with intermediate

radicals can result from experimental uncertainties but

also from deficiencies in the mechanism or inaccuracies in

the kinetic data.

A similar study [185] was carried out by the same

Orleans group but species mole fractions were measured by

gas chromatography so that isomers that could not be

distinguished by the mass spectrometer were identified and

analysed separately. Hence, although the main objective of

this work was to extend the n-heptane combustion

mechanism to atmospheric pressure, it was also to take

advantage of the new data on the isomers to refine the

mechanism. Modifications to the low-pressure mechanism

have been strictly limited to (i) calculation of high pressure

values for reactions in the fall-off regime and (ii) distinction

of the isomeric forms of heptenes. The reliability of the

mechanism was evaluated by comparison of computed mole

fraction profiles with those measured in a rich premixed n-

heptane flame (equivalence ratio of 1.9). Good agreement

was obtained for most molecular species, especially

intermediate olefins, dienes and alkynes while calculated

benzene concentrations were also in reasonable agreement

with experiment. Analysis of the main reaction pathways

show that the main effect of the change of pressure from 6 to

101 kPa is to increase the relative importance of the thermal

decomposition reactions, especially for the intermediate

olefins.

Davis and Law [186] have measured stretch-corrected,

laminar flame speeds of n-heptane–air mixtures at atmos-

pheric pressure and compared their results to three n-

heptane mechanisms [29,175,187]. The Held et al. [187]

mechanism performs best over the complete range of

stoichiometries particularly for f # 0:9:

Ingemarsson et al. [188] investigated an atmospheric

pressure, premixed laminar n-heptane/air flame

using GC–MS sampling to determine species profiles

for a stoichiometric mixture. Key findings of this study are

that alkene concentrations are significantly higher than

corresponding alkanes, that 1-alkene concentrations

decrease with increasing chain length from C2 through

C7, that C4-C7 intermediates peak early whilst C1–C3

peak late, and, that 1,3-butadiene peaks early during the

oxidation of n-heptane. A reaction mechanism for n-

heptane oxidation including thermodata due to Held et al.

[187] was used to model the results with moderately

satisfactory agreement except that several detected species

(propane, propyne, methanol, iso-butene, 2-butene and 1-

heptene) were absent from the model.

The performance of the Lindstedt and Maurice [175],

Held et al. [187] and Curran et al. [189] mechanisms was

also tested by Davidson et al. [190] against n-heptane

ignition delay times, based on emission from methylidyne,

CzH, obtained between 1400 and 1550 K and at reflected

shock pressures of 120 and 220 kPa for mixtures containing

0.4% n-heptane and 4.9% O2 in argon. The Held mechanism

best fits the data and signals that the formation of allyl is

almost as important as Hz þ O2 O O þ OzH in influencing

the ignition delays:

C3H6 þ Hz ! H2CyCH–CzH2 þ H2

Seiser and co-workers [191] studied extinction and

autoignition of n-heptane in a counterflowing non-premixed

system, where transport processes are important, and

modelled the results with a truncated version (770 reversible

reactions of 159 species) of a detailed model of 2540

reactions of 555 species [189]. The truncated model,

necessary because of the larger computational demands of

a heterogeneous configuration, was able to give a satisfac-

tory account of the data showing, inter alia, that high-

temperature chemistry dominates the autoignition process in

the counterflow flame.

High-temperature detailed chemical kinetic reaction

mechanisms were developed by Westbrook et al. [192] for

all nine chemical isomers of heptane, following techniques

and models developed previously for other smaller alkane

hydrocarbon species. These reaction mechanisms were

tested by computing shock-tube ignition delay times for

stoichiometric heptane/oxygen mixtures diluted by argon

[190]. Differences in the overall reaction rates of these

heptane isomers are discussed in terms of differences in

their molecular structure and the resulting variations in

rates of important chain branching and termination

reactions. A similar exercise was carried out [193] in

conjunction with rapid compression machine autoignition

data for 2-methyl hexane, 2,2- and 2,4-dimethyl pentanes

[194]. The computations [195] predict that n-heptane, 2-

methyl hexane and 3-methyl hexane are the most reactive,

Fig. 2, 3-ethyl pentane is less reactive, Fig. 3, and the

remaining isomers are least reactive, Fig. 4. These

observations are only approximately consistent with the

octane rating of each isomer.



time histories during the ignition of stoichiometric n-

heptane/oxygen mixture highly dilute in argon (99.6%)

between 1540 and 1790 K at reflected shock pressures of

200–380 kPa. The results parallel those of propane and

butane by the same Stanford group (see above) and in

comparing three n-heptane models—those due to Lindstedt

and Maurice [175], Held et al. [187] and Curran et al.

[189]—the authors found that none of the three were at all

satisfactory.

Colket and Spadaccini [196] have determined ignition

delay times for n-heptane (and also for ethene and JP-10)

from 1000 to 1500 K, pressures of 300–800 kPa and

equivalence ratios of 0.5–1.5; in a model paper as regards

the presentation of experimental data (experimentalists

please imitate!), they summarise all previous work on n-

heptane.

The Stanford group have also presented [197,198]

ignition time measurements, in reasonably good agreement

with previous measurements by Burcat et al. [199] and

Colket and Spadaccini [196], for the combustion of n-

heptane (also propane, n-butane, and n-decane) behind

reflected shock waves over the temperature range of 1300–

1700 K and pressure range of 100–600 kPa. The test

mixture compositions varied from approximately 0.2–2%

n-heptane, 2–20% O2, and the equivalence ratios ranged

from 0.5 to 2.0. Improved methods of determining the fuel

mole fraction of the test mixture in situ and of measuring the

ignition delay times by a CzH emission diagnostic at the

shock-tube endwall were employed. A parametric study

revealed marked similarity in the ignition delay times of the

four n-alkanes, and they expressed stoichiometric ignition

time data for all four n-alkanes, with a correlation coefficient

of 0.992, as

t ¼ 9:4 £ 10212P20:55xðO2Þ20:63n20:50 expðþ23 245=TÞ

where the ignition time, t; is in seconds, pressure, p; in

atmospheres, xðO2Þ is the mole fraction of oxygen in the test

mixture, and n is the number of carbons atoms in the n-

alkane. The authors present comparisons to past ignition

time studies and detailed kinetic mechanisms to further

validate this correlation. This is an extraordinary result at

first sight since there is no obvious connection between the

four compounds, although all hydrocarbons share common

intermediates and pathways as they react, and, normally the

most important reactions as regards high-temperature

oxidation tend not to be that fuel-specific. Multiple

regression analysis to determine the five parameters in the

global correlation above, can be problematic since

the variables usually span a very limited range, for example

in this case the argon concentration ranges from <80 to

96% but these results are in agreement with work by Toland

[200] who finds that at 1% fuel and either 2 or 4% oxygen,

t(propane) , t(butane), and, at 8% O2 t(propane)

< t(butane), all for pressures of 350 kPa.

Silke [201] has determined the reactivities of eight out of

the nine heptane isomers in a creviced-piston rapid

compression machine study, Fig. 5, and finds that, in

general, the modelling predictions of Westbrook et al. [192]

for the most reactive are borne out, although the correlation

of RON and reactivity for the least reactive isomers is less

clearcut.

2.8. Octanes

Of the 18 isomers only two, n-octane and iso-octane,

have been extensively studied, the most important being

2,2,4-trimethylpentane or iso-octane. A perfectly stirred

reactor study at 923 K by Simon et al. [180] of iso-octane

oxidation with dwell times of 0.1–1.1 s and at sub-

atmospheric pressure determined quantitatively the for-

mation of some 16 products, and, the results modelled with a

generalised mechanism. The formation of hydrogen pre-

sented problems and was not reproducible.

A semi-detailed kinetic scheme from Ranzi and co-

workers [202] is available for iso-octane which simulates

turbulent flow reactor [203], stainless steel and quartz [204]

jet-stirred reactors, rapid compression machine and shock-

tube studies [205] covering the range from 550 to 1500 K

and up 4000 kPa pressure.

Davis and Law [186] determined the stretch-corrected

laminar flame speeds of iso-octane–air mixture over a range

of stoichiometries (their speeds are not in good agreement

with those of Bradley et al. [206]) and modelled the results

with a mechanism assembled from a compact n-heptane

mechanism [187] and an early version of high-temperature

iso-octane chemistry [207]. The model underestimates the

experimental flame speeds by a considerable margin except

for the very leanest mix, f ¼ 0:7; however it does better in

matching the flow reactor experiments of Dryer and

Brezinsky [203] particularly for fuel decay, and major

intermediates such as iso-butene and propene, although

methane and CO are not at all properly represented.

A detailed chemical kinetic mechanism has been

developed and used by Curran et al. [207] to study

Fig. 2. Most reactive heptanes.

Fig. 3. Intermediate reactivity.

Fig. 4. Least reactive heptanes.


the oxidation of iso-octane in a jet-stirred reactor [204], flow

reactors [208,209], shock tubes [210] and in a motored

engine [211] but not against the flame speed measurements

of Davis and Law [186]. Over the series of experiments

investigated, the initial pressure ranged from 100 to

4500 kPa, the temperature from 550 to 1700 K, the

equivalence ratio from 0.3 to 1.5, with nitrogen–argon

dilution from 70 to 99%. This range of physical conditions,

together with the measurements of ignition delay time and

concentrations, provide a broad-ranging test of the chemical

kinetic mechanism which was based on n-heptane oxidation.

Experimental results of ignition behind reflected shock

waves were used to develop and validate the predictive

capability of the reaction mechanism, comprising 3600

elementary reactions of 860 species, at both low and high

temperatures. Moreover, the concentrations of compounds

found in flow and jet-stirred reactors were used to help

complement and refine the low and intermediate tempera-

ture portions of the reaction mechanism, leading to good

predictions of intermediate products in most cases. In

addition, a sensitivity analysis was performed for each of the

combustion environments in an attempt to identify the most

important reactions under the relevant conditions of study.

Davidson et al. [212] have carried out measurements of

ignition delay times and OzH concentration profiles for iso-

octane at 1180–2010 K, 120–820 kPa and for mixtures

with 0:25 # f # 2: The OzH time history differs quite

markedly from those this group measured for n-alkanes

[151] featuring a drop-off just prior to ignition.

Detailed modelling of the oxidation of n-octane (and n-

decane) in the gas phase was performed by Glaude and co-

workers [213] using computer-designed mechanisms. For n-

octane, the predictions of the model were compared with

experimental results obtained by Dryer and Brezinsky in a

turbulent plug flow reactor [203] at 1080 K and 100 kPa.

Considering that no fitting of any kinetic parameter was

done, the agreement between the computed and the

experimental values is satisfactory both for conversions

and for the distribution of the products formed. This

modelling has required improvement in the generation of

the secondary reactions of alkenes, which are the main

primary products obtained during the oxidation of these two

alkanes in the range of temperature studied and for which

reaction paths are detailed.

2.9. Decanes

Dagaut et al. [214] have modelled the oxidation of n-

decane in a jet-stirred reactor at 1 MPa pressure, from 550 to

1150 K, at residence times of 0.5 and 1 s, and for 0:1 #

f # 1:5; their detailed mechanism gives a good description

for species profiles at temperatures over 800 K but does not

match the data that well in the intermediate to low

temperature regions.

The chemical structure of a premixed n-decane/O2/

N2 flame ðf ¼ 1:7Þ stabilized at atmospheric pressure on a

flat-flame burner has been computed with two reaction

mechanisms by Doute et al. [215]. In the first one,

Fig. 5. Ignition delay times at post-compression pressure of 1.5 MPa except heptane at 2 MPa: (A) heptane (0), (S) 2-methylhexane (42.4), (K)

3-methylhexane (52.0), 3,3-dimethylpentane (80.8), (L) 2,4-dimethylpentane (83.1), (W) 2,3-dimethylpentane (91.1), (N) 2,2-dimethylpentane

(92.8), ( M ) 2,2,3-trimethylbutane (112); research octane numbers in brackets.


the consumption of the fuel molecule is described in detail.

The five different n-decyl radicals formed by H atom

abstraction from the decane molecule were distinguished

and their consumption reactions were considered in a

systematic way. This mechanism comprises 78 species

involved in 638 elementary reactions, modelling with this

detailed mechanism led to species mole fraction profiles in

good agreement with the experimental results. The main

reaction paths for the formation of final and intermediate

species have been identified with special emphasis on

benzene formation. The second mechanism was derived

from the first one by successively removing an increasing

number of n-decyl radicals. For most species, it is possible

to maintain the reliability of the model with only one n-

decyl radical in the mechanism-an example of the successful

adoption of the ‘principle of shortsightedness’ [216]. In this

simplified version of the mechanism, the species number is

reduced to 62 and the reaction number to 467. The only

species affected are the large intermediate olefins.

Detailed modelling of the oxidation of n-decane was

carried out by Glaude et al. [213] with an automatically

generated mechanism and compared with the jet-stirred

reactor species profiles data of Bales-Gueret et al. [217]

obtained at temperatures of 922–1033 K, residence times of

0.1–0.25 s and atmospheric pressure.

Battin-Leclerc and co-workers [218] have simulated n-

decane experiments performed in a jet-stirred reactor [217]

and in a premixed laminar flame [219] from 550 to

1600 K. Their mechanism, generated automatically,

included a massive 7920 reactions.

Zeppieri et al. [220] have developed a partially reduced

mechanism for the oxidation and pyrolysis of n-decane

validated against flow reactor, jet-stirred reactor [217] and

n-decane/air shock-tube ignition delay [29] data. The

approach includes detailed chemistry of n-decane and the

five n-decyl radicals, and it incorporates both internal

hydrogen isomerization reactions and b-scission pathways

for the various system radicals. To include this additional

detailed reaction information and simultaneously minimize

the number of species present in the model, an important

assumption was made regarding the distribution of radical

isomers. It was assumed that the different isomers of a given

alkyl radical are in equilibrium at each carbon number

above the C4 level, thereby allowing the inclusion of the

reaction channels associated with each isomer, without

imposing the computational penalty associated with includ-

ing each isomer as a separate species in the mechanism. As a

result, only a single radical is needed to represent all the

isomers associated with it. Thus, the new mechanism

contains detailed reaction chemistry information, while

maintaining the compactness necessary for use in combined

fluid mechanical/chemical kinetic computational

simulations.


time histories during the ignition of a stoichiometric n-

decane/oxygen mixture highly dilute in argon ($96.7%)

between 1350 and 1700 K at reflected shock pressures of

220 kPa. The results parallel those of propane, butane and n-

heptane by the same Stanford group (see above); the n-

decane oxidation mechanism of Lindstedt and Maurice

[175] does not give a satisfactory account of the OzH profile

whilst the Battin-Leclerc et al. mechanism [218] could not

be run because of its large size.

A chemical kinetic mechanism for the combustion of n-

decane has been compiled and validated by Bikas and Peters

[221] for a wide range of combustion regimes. Validation

has been performed by using measurements on a premixed

flame of n-decane, O2 and N2, stabilized at 100 kPa on a flat-

flame burner [215], as well as from experiments in shock

waves [222], in a jet-stirred reactor [223] and in freely

propagating premixed flame [224]. The reaction mechanism

features some 600 reactions and 67 species including

thermal decomposition of alkanes, H-atom abstraction,

alkyl radical isomerization, and decomposition for the

high temperature range, and a few additional reactions at

low temperatures. The transition between low and high

temperatures with a negative temperature dependence is

quite well reproduced.

Ignition delay times from 1260 to 1560 K, 500–

1000 kPa and 0:5 # f # 1:5; and, flame speeds in an

atmospheric pressure, laminar, premixed flame 0:9 # f #

1:3 of n-decane have been determined by Skjøh-Rasmussen

et al. [225] and compared with the predictions of a number

of decane mechanisms [213,220,221,223] with only the

classical detailed mechanism of Dagaut et al. [223] in

agreement with the measured flame speeds; the ignition

delays are not matched at all well by any of the models.

2.10. Higher hydrocarbons

A modelling study by Ristori et al. [226] of the oxidation

of a key diesel fuel component, n-hexadecane or cetane, was

based on experiments performed in a jet-stirred reactor, at

temperatures ranging from 1000 to 1250 K, at 100 kPa

pressure, a constant mean residence time of 70 ms, and a

high degree of nitrogen dilution (0.03 mol% of fuel) for

equivalence ratios equal to 0.5, 1, and 1.5. The kinetic model

features 242 species and 1801 reactions and gives

reasonable agreement with species profiles except, some-

what surprisingly, for the parent fuel, n-C16H34 itself whose

reactivity is underestimated. In a parallel paper based on the

same experiments a detailed kinetic mechanism [227] was

automatically generated [228] by using the computer

package, EXGAS, developed in Nancy. The long linear

chain of this alkane necessitates the use of a detailed

secondary mechanism for the consumption of the alkenes

formed as a result of primary parent fuel decomposition.

This high-temperature mechanism includes 1787 reactions

and 265 species, featuring satisfactory agreement for the

formation of products but still does not adequately account

for the consumption of hexadecane.


2.11. Cyclics, rings

Ring systems may well exhibit dramatically different

mechanisms not just from linear analogues but also with

each other.

2.11.1. Three and four

Monocyclic small ring hydrocarbons do not appear to

have been much studied; Slusky et al. [229] have

determined the ignition delays, t; of cyclopropane and

cyclobutane (as well as substituted derivatives and

bicyclic species) in stoichiometric air-like argon mixtures

between 1200 and 1600 K and at reflected shock pressures

of 600 ^ 100 kPa. Interestingly they find that cyclopro-

pane is less reactive than cyclobutane, that is, t(cC3H6) .

t(cC4H8), although the latter mixture contains more

oxygen and since normally t/ ½O2�21 it is not possible

to get a true comparison. The authors argue from known

rates of isomerisation that the transformation of cyclopro-

pane to propene is completed long before ignition occurs

but then present data which shows that t(propene) <1.8 £ t(cyclopropane); this is not consistent. A similar

argument is made for cyclobutane and its decomposition

product of two ethylene molecules.

2.11.2. Five

An experimental study of the oxidation of cyclopentane

(and n-pentane) was performed at 873 K in a jet-stirred

reactor by Simon et al. [171] with residence times of 0.1–

0.5 s at 53 kPa corresponding to between 2 and 24% fuel

consumption for a rich mixture with f ¼ 2; they discuss

species profiles in terms of a generalised mechanism.

Laminar premixed flat cyclopentene/oxygen/argon

flames with different stoichiometries (C/O ¼ 0.6, 0.77,

and 0.94) were studied by Lamprecht et al. [230] at 5 kPa

under fuel-rich non-sooting conditions motivated by the

scarcity of information on C5 fuel combustion. Concen-

trations as a function of height above the burner were

measured for more than 30 stable and radical species using

molecular beam mass spectrometry. Temperature was

measured in the unperturbed flame with laser-induced

fluorescence of seeded NO. Stable species concentrations

in the burned gases were found in good agreement with

equilibrium calculations. For information on the flame

structure in the reaction zone, species profiles for inter-

mediates of relevance in the formation of aromatics were

inspected regarding in particular several CxHy compounds

with 2 # x # 10: The measured data was analysed with

respect to the formation of C6 species, in particular of

benzene as a key species in the soot formation mechanism.

A reaction flow analysis has been performed which reveals

striking differences to other fuels, including acetylene and

propene. It does not seem feasible to rely on a single

dominant pathway to benzene in cyclopentene flames.

Reactions of C5H5 and C5H6 were found to be important,

that of C5H6 þ CH3 being of similar influence on C6H6

formation as the propargyl recombination, a result of

interest for detailed flame modeling, which was however

not carried out.

Cyclopentadiene ignition was measured by Burcat et al.

[231] in a study from 1280 to 2110 K, 240–1250 kPa and

with f ¼ 0:5–2; as usual there is very little dependence

of ignition delay on fuel concentration and the normal

reciprocal dependence on [O2]. In addition some exper-

iments were performed to determine intermediate concen-

trations prior to ignition; the main products apart from CO

are acetylene and benzene (in surprisingly large amounts).

The detailed model of 439 reactions could be reduced to a

skeletal 125 and still represent the experiments except for

the leanest mixtures. Cyclopentadiene combustion is

important because of the light that it can throw on

benzene oxidation since phenoxy decomposes to

cyclopentadienyl.

2.11.3. Six

Cyclohexane oxidation has been studied by Voisin et al.

[232] in a jet-stirred reactor in the temperature range of

750–1100 K at 1000 kPa. Major and minor species profiles

have been obtained by probe sampling and GC analysis. A

chemical kinetic reaction mechanism developed from

previous studies on smaller hydrocarbons is used to

reproduce the experimental data. It has been updated and

validated for C1–C5 sub-mechanisms. Good agreement is

obtained between computed and measured mole fractions.

The major reaction paths of cyclohexane consumption and

the formation and the consumption routes of the main

products have been identified for the experimental con-

ditions with the formation of ethylene identified as the key

thermal decomposition step: C6H12 ! 3H2CyCH2.

In an extension of this work to lower pressures (but

similar temperatures) and with an improved analytical

technique for the detection of intermediates, a detailed

reaction mechanism [233] for cyclohexane oxidation has

been evaluated by comparison of computed and exper-

imental species mole fraction profiles measured in a jet-

stirred reactor for f ¼ 0.5–1.5 and 100, 200, and

1000 kPa. Major and minor species mole fractions were

obtained for O2, CO, CO2, H2, H2CO, CH3CHO,

H2CyCH–CHO or acrolein, CH4, C2H6, C2H4, C3H6,

C2H2, allene, propyne, 1-C4H8, 2-C4H8 (both trans and

cis), butadiene, cyclopentene, cyclohexadiene, 1-hexene,

cyclohexene, and C6H6. Good agreement was obtained for

most molecular species, especially intermediate olefins,

dienes, and oxygenated species such as H2CO and

acrolein. This mechanism assumes thermal decomposition

of the cyclohexane to ethylene and cyclobutane initially,

although cyclobutane is not observable.

Computed benzene and cyclopentene concentrations are

in reasonable agreement with experimental data but

cyclohexene and 1,3-cyclohexadiene are over-predicted.

The mechanism, comprising 107 species and 771 reactions,

was also validated at higher temperature by modelling


the laminar flame speeds of cyclohexane–air flames

measured by Davis and Law7 [234] over a wide range of

equivalence ratios, although there is not much sensitivity

exhibited to reactions involving large ring fragments except,

oddly, for phenoxy, C6H5Oz

The oxidation of n-propylcyclohexane has been

studied by Ristori et al. [235] at atmospheric pressure

in a jet-stirred reactor from 950 to 1250 K with

0.5 # f # 1.5; concentration profiles of reactants, inter-

mediates and final products, formed during the 70 ms

reaction time, were determined. The detailed model of

176 species and 1369 reactions attempts to trace some of

the main pathways which they show proceeds via H-

atom abstraction to form seven propylcyclohexyl

radicals that react by b-scission to yield

ethylene, propene, methylenecyclohexane, cyclohexene

and 1-pentene, Fig. 6.

Principal decomposition channels involve the parent

propylcyclohexane pyrolysing to:

3CH2CH2 þ CH3CHCH2

2CH2CH2 þ CH3ðCH2Þ2CHCH2

CH2CH2 þ CH3CH2CzH2 þ CH2CHCH2CzH2

as well as some additional channels involving bond-

breaking reactions.

2.11.4. Multiple rings

Recent interest in exo-tetrahydrodicyclopentadiene has

been driven by the potential of this high-energy

compound, for use in scramjets and pulse detonation

engines, since it is the principal component of the jet fuel

JP-10, Fig. 7. Williams and co-workers [236,237] have

established a partially detailed mechanism with 174

reactions, not all of which are elementary, dealing with

36 species. In essence this study utilises the ignition times

and OzH concentration time histories for JP-10/O2/Ar

mixtures measured behind reflected shock waves over the

temperature range of 1200–1700 K, pressure range of

100–900 kPa, fuel concentrations of 0.2 and 0.4%, and

stoichiometries of 0.5, 1.0, and 2.0 by Davidson et al.

[238]. Colket and Spadaccini [196] have also reported

autoignition data for JP-10 (as well as ethylene and n-

heptane) at 1100–1500 K, 300–800 kPa and equivalence

ratios of 0.5–1.5 but did not carry out any detailed

modelling. Neither did Olchansky and Burcat [239] who

have determined ignition delay times as well but also

sampled species formed between 1050 and 1135 K finding

inter alia cyclopentene, pentadiene, ethene, benzene,

butadiene, butenes, propene, toluene, CO, etc. nor

Mikolaitis et al. [240] who have reported JP-10/air

ignition delay times at reflected shock pressures of 1–

2.5 MPa and temperatures from 1200 to 2500 K.

An earlier kinetic model for JP-10 oxidation used

global decomposition reactions proposed by Williams et al.

[241] in conjunction with a larger alkane mechanism of

Lindstedt and Maurice [175]. This modelling gave good

agreement with the ignition times at higher pressures, and

sensitivity studies using this model indicated the important

role of C2 chemistry in JP-10 decomposition. However,

as the authors acknowledge, the mechanism is

incomplete and is not good at describing the early

stages of fragmentation and oxidation of tetrahydrodicy-

clopentadiene.

Some earlier work compared the ignition delay times for

stoichiometric air (with the nitrogen replaced by argon)

mixtures of spiropentane, hexane and heptane [229,242] at

1100 – 1600 K and reflected shock pressure of

600 ^ 100 kPa. The reactivities increase from spiroheptane

through spiropentane to spirohexane, Fig. 8, with the high

reactivity of the C6 compound being ascribed to the prompt

formation of ethylene and butadiene which then react with

oxygen.

3. Alkenes and dienes

3.1. Ethene

The simplest alkene, ethene or ethylene, H2CyCH2, has

been the subject of numerous combustion experiments and

associated modelling studies, many of them dealing with the

formation of soot particles—these will not be dealt with

here.

Experiments and detailed modelling has been per-

formed by Castaldi et al. [243] to investigate the mono

and polycyclic aromatic formation pathways in premixed,

rich ðf ¼ 3:06Þ; sooting, ethylene–oxygen–argon burner

Fig. 6. Abstraction by O2 from C9H18 and formation of

methylenecyclohexane.Fig. 7. Exo-tetrahydrodicyclopentadiene or JP-10.

Fig. 8. Spiro compounds.

7 This paper also contains reliable flame speed data for propene,

butanes, butenes, 1,3-butadiene, n- and cyclopentane, n-hexane,

benzene and toluene.


stabilised atmospheric-pressure flame. Species detected in

the flame and post-flame regions included allene

and propyne, diacetylene, vinylacetylene, 1,2- and 1,3-

butadienes, 1- and 2-butynes, 1- and 2-butenes, cyclopen-

tadiene, toluene, ethylbenzene, styrene, phenylacetylene,

o-xylene, indene, methylnaphthalene, acenaphthalene,

biphenylene, biphenyl, cyclopenta[cd ]pyrene and benzo[-

ghi ]fluoranthene. The detailed model of 664 reactions of

150 species struggles to match the experimental data.

A detailed chemical kinetic mechanism, including 340

elementary steps and 90 species, has been developed by

D’Anna and Violi [244] to simulate the formation of

aromatic compounds in rich premixed flames of aliphatic

hydrocarbons. The mechanism can reproduce the concen-

tration profiles and net rates of benzene and larger aromatic

hydrocarbons (two- and three-ring polycyclic aromatic

hydrocarbons (PAHs)) in a wide range of temperatures for

a slightly sooting, premixed ethylene–oxygen flame. Key

sequences of reactions in the formation of aromatics are the

combination of resonantly stabilized radicals, whereas the

alternative mechanism that involves acetylene addition does

not seem to be fast enough to explain the observed

formation rates of aromatics in the flames examined. The

main routes involved in the formation of the first aromatic

ring are the propargyl self-combination and its addition to 1-

methylallenyl radicals. Cyclopentadienyl radical combi-

nation, propargyl addition to benzyl radicals, and the

sequential addition of propargyl radicals to aromatic rings

are the controlling steps for the formation of larger aromatic

species.

Pyrolysis and oxidation of ethylene was studied behind

reflected shock waves by Hidaka et al. [245] in the

temperature range 1100–2100 K and at pressures of 150–

450 kPa. Ethylene decay in both the pyrolysis and

oxidation reactions was measured by absorption at

3.39 mm and emission at 3.48 mm. CO2 production was

also measured by time-resolved IR-emission at 4.24 mm

while species yields were also determined by the single

pulse sampling. The pyrolysis and oxidation of ethylene

were modeled using a kinetic reaction mechanism of 161

reactions and 51 species, including the most recent

mechanism for formaldehyde, ketene, methane, ethane

and acetylene oxidations.

Wang [246] has explored the importance of carbenes as

free-radical chain initiators in the oxidation of ethylene (also

propyne and allene) by combined DFT calculations and

kinetic modelling of ignition delay measurements [245,247,

248] and concludes that the carbene pathway dominates the

initial radical pool production.

Ethylene ignition and detonation was modelled by

Varatharajan and Williams [249] for a series of shock-

tube measurements covering the ranges 1000–2500 K,

50–10,000 kPa and equivalence ratios between 0.5 and 2

and usefully summarised in the paper. Their mechanism of

148 elementary reactions involving 34 species is in good

agreement with experimental burning velocities for

laminar ethylene flames (although the measurements

were not stretch-free) and with the shock-tube induction

times; unsurprisingly, the mechanism performs better than

GRI-Mech 3.0.

Ethene combustion has been modelled by Carriere et al.

[250] using data from the Princeton flow reactor from 850 to

950 K, 500–1000 kPa and f ¼ 2:5; and, for a premixed,

low-pressure (2.7 kPa), laminar, fuel-rich ðf ¼ 1:9Þ flame

[251]. In the low-pressure flame ethene is consumed mainly

by abstraction reactions, to H2CyCzH, whilst in the flow

reactor abstraction by OzH competes with H-addition to

H3CCzH2. Their reaction mechanism of 737 reactions (of

which 641 were reversible) and 86 species was judged to

compare favourably with experiment and thereby to perform

appreciably better than mechanisms by Dagaut et al. [252],

GRI-Mech 3.0 [58] and Wang and Laskin [253].

3.2. Propene

Propene oxidation chemistry in laminar premixed

flames was studied by Thomas et al. [254] in a lean ðf ¼

0:229Þ; low pressure laminar premixed C3H6–O2–Ar flame

and comparisons with experiment made in order to assess

existing uncertainties in the propene oxidation chemistry. It

is shown that propene is mainly consumed by O atom

addition reactions. However, reactions involving the OzH

radical remain important and a tentative branching ratio

between abstraction and addition channels for the OzH

attack on propene is also proposed. Furthermore, it has

been shown that computed allyl radical levels are sensitive

to the choice of rate for the molecular oxygen attack and a

tentative product distribution for the latter is also proposed.

Generally good agreement is obtained between compu-

tations and measurements for major flame features and key

combustion intermediates. Moreover, allyl radical and total

C3H4 levels are successfully reproduced. A tentative

reaction mechanism for C-3 oxygenated species has also

been formulated and validated against experimental data.

Finally, the study identifies specific aspects of the propene

oxidation chemistry where further theoretical and exper-

imental work is required.

The pyrolysis and oxidation of propene or propylene was

studied experimentally in an atmospheric-pressure plug flow

reactor with residence times of 4–180 ms by Davis et al.

[255]. Species profiles were obtained in the intermediate to

high temperature range (close to 1200 K) for lean,

stoichiometric, rich, and pyrolytic conditions; only one

oxygenated product, apart from the obvious CO, CO2 and

H2O, was detected but not identified. Laminar flame speeds

of propene/air mixtures were also determined over an

extensive range of equivalence ratios (0.7–1.7), at room

temperature and atmospheric pressure, using the counter-

flow twin flame configuration.

A detailed chemical kinetic model consisting of 469

reactions and 71 species was used to describe the high-

temperature kinetics of propene; the authors also discuss


flow reactor species profiles, laminar flame speeds and

ignition delays for propyne oxidation, shock-tube ignition

for allene, and laminar flame speeds and ignition delays for

propane combustion. The kinetic model is in good

agreement with the measured stretch-compensated laminar

premixed propene flame speeds for f # 1 but under-

estimates flame speed quite strongly above this point The

model predicts much shorter values of ignition delay than

those determined by Burcat and Radhakrishnan [256], but

these latter results have been called into question.

Sensitivity studies reveal that the following reactions, as

well as the usual Hz þ O2 ! OzH þ O, are most important

in the oxidation of propene at 1200 K:

H2CyCH–CzH2 ! H2CyCyCH2 þ Hz

H3C–CHyCH2 þ O2 ! H2Cz –CHyCH2 þ HOz

2

H2Cz –CHyCH2 þ HOz

2 ! H2CyCzH þ OyCH2 þ OzH

Propene ignition was studied behind reflected shock

waves by Qin et al. [257] at postshock temperatures ranging

from 1270 to 1820 K and postshock pressures from 95 to

470 kPa, when reactant concentrations were varied from 0.8

to 3.2% propene and from 3.6 to 15.1% oxygen diluted in

argon, giving equivalence ratios ranging from 0.5 to 2.0.

The measurements were not in good agreement with the

previous results of Burcat and Radhakrishnan [256]. The

data could be accounted for using a reaction mechanism

with 463 elementary reactions [141].

The computer code EXGAS was used to generate

detailed mechanisms for the oxidation and combustion of

alkenes [258]. An analysis of the elementary reactions

from the literature allowed the definition of new specific

generic reactions involving alkenes and their free radicals,

as well as correlations to estimate the related rate

constants. The corresponding generic rules were then

implemented in the EXGAS code and a mechanism for the

oxidation of propene involving 262 species and including

1295 reactions was generated. The predictions of this

mechanism were compared with two sets of experimental

measurements: the first obtained in a static vessel

between 580 and 740 K; the second used a jet-stirred

reactor between 900 and 1200 K. If one takes into

account that no fitting of individual rate constants was

done, the mechanism reproduces correctly both the NTC

observed at ,630 K and the variations of the concen-

trations with residence time of C3H6, CO, CO2, CH4,

C2H2, C2H4, C3H4, H2CO, CH3CHO, CH2CHCHO, and

cyclic ethers, especially the general shape of these curves

and their minima, maxima, and inflection points. In a

later paper [259] they use the same mechanism to match

experimental ignition delays for propene obtained in

shock tubes [256,257].

Flux and sensitivity analyses were performed to get

insight into the kinetic structure of the mechanism

explaining the observed characteristics, such as the NTC

or the autocatalytic behaviour of the reaction. At low

temperatures, these analyses showed that the NTC is mainly

due to the reversibility of the addition of oxygen to the

adducts which yield degenerate branching agents. At high

temperatures, in both kind of reactor, the determining role of

termination reactions involving the very abundant allyl

radicals has been emphasized, especially the recombination

of allyl and hydroperoxy radicals, which is the main source

of acrolein.

Reactivity experiments on propene oxidation at 1.3 MPa

from 500 to 860 K in a variable pressure flow reactor were

carried out by Zheng et al. [260] as well as species times

histories for reactants, intermediates and products. A

detailed mechanism based on propane work by Qin et al.

[141] gave a better account of the experiments than the

Heyberger et al. model [258], including correctly predicting

the absence of an NTC region.

3.3. Butenes

Iso-butene oxidation and ignition has been studied by

Dagaut and Cathonnet [261] in a jet-stirred reactor from 800

to 1230 K and at 100, 500 and 1000 kPa pressure; their

results for species concentration profiles and earlier ignition

delay measurements of Curran et al. [262,263] were

modelled with a 110 species, 743 reactions mechanism.

The species profiles are reasonably well simulated although

2-methyl-1 and 2-methyl-2-butenes, acrolein and isoprene

are underpredicted.

Bauge et al. [264] describe an experimental and

modelling study of the oxidation of iso-butene. The low-

temperature oxidation was studied in a continuous-flow

stirred-tank reactor operated at constant temperature (from

833 to 913 K) and 100 kPa pressure, with fuel equivalence

ratios from 3 to 6 and space times ranging from 1 to 10 s

corresponding to iso-butene conversion yields from 1 to

50%. The ignition delays of iso-butene–oxygen–argon

mixtures with 1 # f # 3 were measured behind shock

waves from 1230 to 1930 K and pressures from 950 to

1050 kPa. A mechanism is able to reproduce moderately

well the profiles obtained for the reactants and the

products during the slow oxidation; however, the stoi-

chiometric ignition delays are not well fitted. The main

reaction paths have been determined for both series of

measurements by both sensitivity and rate of production

analysis.

The lean oxidation of iso-butene has been studied in a

high pressure, plug flow reactor by Chen et al. [265] at

920 K and 600 kPa; the concentrations of 15 intermediates

were quantified and compared to the predictions of a

mechanism comprised of 3570 reactions of some 850


species on the assumption of isobaric plug flow with no axial

diffusion of species or energy. Overall the mechanism

underestimated the concentrations of most of the

intermediates.

Chen and Bozzelli [266] have analysed the kinetics for

the reactions of allylic isobutenyl radical H2CyC(CH3)CzH2

with molecular oxygen by using quantum Rice–Ramsper-

ger–Kassel theory for kðEÞ and master equation analysis for

falloff. Thermochemical properties and reaction path

parameters were determined by ab initio and density

functional calculations and an elementary reaction mech-

anism constructed to model experiments [267] in iso-butene

oxidation.

Heyberger et al. [259] have modelled the jet-stirred

reactor oxidation [268] and ignition in shock waves of 1-

butene with 377 reactions of 180 species valid only above

900 K. The predictions of the mechanism produced for the

oxidation of 1-butene compared successfully with both

sets of experimental results: the first obtained in a jet-

stirred reactor between 900 and 1200 K; the second being

new measurements of ignitions delays behind reflected

shock waves from 1200 up to 1670 K, pressures from 670

to 900 kPa, equivalence ratios from 0.5 to 2, and with

argon as bath gas. Flux and sensitivity analyses show that

the role of termination reactions involving the very

abundant allylic radicals is less important for 1-butene

than for propene.

3.4. Higher alkenes and dienes

The autoignition of 1-pentene has been studied by

Ribaucour et al. [164] in a rapid compression machine

between 600 and 900 K and at 600–900 kPa. The main

features are an ignition limit at ,700 K, a cool flame region

between 700 and 800 K and an NTC near 760–800 K; 1-

pentene is less reactive than pentane under the same

conditions. The authors discuss their results in terms of a

generalised mechanism of 888 reactions between 179

species and focus attention on the formation of cyclic

ethers of which propyloxirane is predominant.

A fuel-rich, non-sooting 1-pentene–oxygen–argon

low-pressure flame was studied by Alatorre et al.

[269] who measured species profiles mass spectroscopi-

cally and modelled the results; the authors conclude that

pentene consumption to propene and ethene is the

dominant reaction and draw parallels with their pre-

vious work on propene–oxygen–argon flames [270]

whilst benzene formation is governed by propargyl

radical recombination.

A low-temperature oxidation and modelling study of

cyclohexene by Ribaucour et al. [271] explored the

temperature range from 650 to 900 K and pressures of

760–1580 kPa and measured autoignition delay times in a

rapid compression machine compared with the predictions

of a mechanism comprising 136 species and 1024

reactions.

The oxidation of allene, CH2CCH2, has been studied by

Pauwels et al. [272] at 1% concentration in a rich, low-

pressure hydrogen/oxygen/argon flame and stable species

profiles and OzH concentrations measured. Their detailed

kinetic model, based on earlier work by Miller and Melius

[273], identifies the reaction of C3H2 with oxygen as of

paramount importance:

HCxCCzH þ O2 ! HCxCOz þ CO þ Hz

The oxidation of 1,3-butadiene has been investigated by

Dagaut and Cathonnet [274] in a jet-stirred reactor at high

temperature (750–1250 K), variable pressure (100 and

1000 kPa) and variable equivalence ratio ð0:25 # f # 2Þ:

Molecular species concentration profiles forO2, H2, CO,CO2,

H2CO, CH4, C2H2, C2H4, C2H6, C3H4, C3H6, acrolein, 1- and

2-butenes, butadiene, vinylacetylene, cyclopentadiene, and

benzene were obtained by probe sampling and GC analysis.

The oxidation of butadiene was modelled using a detailed

kinetic reaction mechanism (91 species and 666 reactions,

most of them reversible) which is able to predict the

experimental results reasonably well. Sensitivity analyses

and reaction path analyses, based on species net rate of

reaction,areusedto interpret theresults.Theroutes tobenzene

formation have been delineated: At low fuel conversion and

low temperature, benzene is mostly formed through the

addition of vinyl radical to 1,3-butadiene, yielding 1,3-

cyclohexadiene, followed by two channels: (a) elimination of

molecular hydrogen to yield benzene and (b) decomposition

of 1,3-cyclohexadiene yielding cyclohexadienyl followed by

its decomposition into benzene and H atom; at high fuel

conversion and higher temperature, (c) the recombination of

propargyl radicals and (d) the addition of vinyl to vinylace-

tylene increasingly yield to benzene formation.

Fournet et al. [275] have determined ignition delays for

1,3-butadiene (also acetylene, propyne and allene) at 1000–

1650 K and reflected shock pressures of 850–1000 kPa;

under similar conditions they find that acetylene is the most

reactive followed by butadiene and with allene as reactive as

propyne. Their model was also compared to stable and

radical species profiles for laminar premixed butadiene

flames [276] and acetylene flames [277].

Laskin et al. [152] have studied the high-temperature

oxidation of 1,3-butadiene in a flow reactor at 1035–1185 K

and at atmospheric pressure. Their kinetic model of 92 species

and 613 reactions [278] was validated against shock-tube

ignition data [275], laminar flame speeds [234] and pyrolytic

work. Three separate pathways for butadiene oxidation were

identified with the chemically activated reaction of H-atom

with butadiene producing ethylene and vinyl, Fig. 9, radical

Fig. 9. H-atom addition to butadiene.


being the most important channel of consumption (once the

radical pool is established) than the O-atom addition, Fig. 10,

over all experimental conditions.

The model is a good fit to the flow reactor data, albeit

that the latter are time-shifted, but does not match the

ignition delay data that well and it underpredicts the

maximum flame speed by ,4 cm s21.

4. Alkynes

4.1. Ethyne

Peeters and Devrient [279] reacted ethyne or acetylene,

HCxCH, and oxygen with O and H-atoms in a fast-flow

reactor at 600 K, at a total pressure of 270 kPa and flow

times of 1–5 ms. Mass spectrometric sampling determined

the concentrations of reactive species such as CzH, CH2,

HCzCO and HCCz and compared these with a detailed

kinetic model which could be simplified by using the

measured reactant concentration profiles for C2H2, Oz, Hz

and O2 as a given. An additional experiment in which the

usual diluent helium was replaced by methane served to

confirm the good agreement between the model and

experiment.

Hidaka and co-workers [280] studied the oxidation

(and pyrolysis) of acetylene behind reflected shocks from

1100 to 2000 K at pressures of 110–260 kPa by analysing

reacted gas mixtures and from measurements of ignition

delay times. The mixture compositions ranged from 0.5 to

4.0% for ethyne, 0.4–5.0% for oxygen with the balance

argon. Existing mechanisms did not give a good fit to the

data and so they propose a 103 reaction/38 species model

which does.

Ryu et al. [281] studied the detonation characteristics of

ethyne behind reflected shock waves from 800 to 1350 K

over a very wide range of mixture compositions

(C2H2:O2:Ar ¼ 2–10:5–32:60–93%) and simulated their

data with a mechanism of 33 reactions based on earlier work

by Hidaka et al. [282]. They identify the formation of formyl

as a key step:

HCCH þ O2 ! HCzyO þ HCzyO

They report that their ignition delay times have

essentially zero dependence on oxygen, t/ ½O2�20:1 and a

high dependence on argon, t/ ½Ar�1:33; these are not in

accord with the Hidaka data, Fig. 8 of [280].

Fournet et al. [275] have measured ignition delays of

ethyne (allene, propyne and 1,3-butadiene were also studied

under identical conditions) from 1010 to 1380 K and

pressures of 850–1000 kPa for three different mixtures

whose fuel:oxygen:argon ratios were 1:4:95, 1:8:91 and

3:12:85. As expected the ignition delay times decrease with

increasing O2 content; acetylene is the most reactive fuel

followed by butadiene and then propyne ¼ allene (all at a

constant O2), Fig. 11.

Laskin and Wang [283] analysed the reaction

between molecular oxygen and acetylene and concluded

that isomerisation to vinylidene precedes reaction.

Detailed kinetic models which included this initiation

process did match experimental [280,282] ignition delays

quite well.

The exact routes traced by the direct reaction between

acetylene and oxygen have been computed by Sheng and

Bozzelli [284] who conclude that at 1000 K and high-

pressures Laskin and Wang’s isomerisation pathway is

viable:

HCCH ! H2CC : !O2 3CH2 þ CO2

but that another initiation also contributes to acetylene

oxidation, namely:

HCCH þ 3O2 !3HCzCHOz

2

V 1HCzCHOz

2 ! 2HCzO or Hz þ OzCCHO

Ethyne oxidation has been addressed numerically with a

114 step mechanism involving 28 species by Varatharajan

and Williams [285] who also usefully summarise ignition

delay times measured over the years in shock waves.8 At

high temperatures the ketyl radical, HCzyCyO, dominates

with the vinyl radical, H2CyCzH becoming more important

at lower temperatures.

4.2. Propyne

The ignition and oxidation of propyne (and allene) has

been studied by Curran et al. [286] in a single-pulse shock-

tube from 800 to 2030 K, 200–500 kPa and for f ¼

0:5 ! 2; and, in a jet-stirred reactor from 800 to 1260 K, 100

and 1000 kPa pressure and f ¼ 0:2 ! 2: A detailed model

provides good agreement for the ignition delay times and for

the concentration profiles, obtained with a range of

residence times from 20 to 2.4 s in the reactor, for a wide

range of intermediates.

Propyne or methylacetylene, H3C–CxCH, has been

studied by Davis and co-workers [287] in the Princeton

turbulent flow reactor at atmospheric pressure, at

,1170 K and for a number of stoichiometries (0.7, 1.0

and 1.4). In addition they measured the laminar flame

speeds from f ¼ 0:7–1:7 in nitrogen–diluted air using

the counterflow twin flame technique with corrections for

flame-stretch effects. Their detailed kinetic model of 69

species and 437 reactions [288] provides good agreement

not just for the flow reactor data but also for ignition

Fig. 10. O-atom addition to butadiene.

8 But not those of Ref. [281].


delay times obtained previously by Curran et al. [286]; the

comparison of flame speeds shows that the model predicts

faster flame speeds (,2 cm s21) than that observed, fairly

uniformly over the whole range of equivalence ratios

used. Only the following reactions, involving C3 species,

influence the computed flame speed to any extent:

H3C–CxCH þ Hz ! H3Cz þ HCxCH

Cz

3H3 þ OzH ! Cz

3H2 þ H2O

Fournet et al. [275] have measured ignition delays of

propyne and allene (acetylene and 1,3-butadiene were

also studied under identical conditions) from 1200 to

1720 K and pressures of 850–1000 kPa for three different

mixtures whose fuel:oxygen:argon ratios were 1:4:95,

1:8:91 and 3:12:85. As expected the ignition delay times

decrease with increasing O2 content; as found previously

[286], the behaviour of allene (technically not an alkyne)

is hardly distinguishable from that of methylacetylene.

Modelling the results shown in Fig. 11 with Konnov’s

mechanism [69] gives good qualitative agreement

for propyne, allene and butadiene except that

the mechanism predicts 10-fold slower ignition delay

times but there is good quantitative agreement for

acetylene [289].

Data for propyne and allene oxidation was obtained by

Faravelli et al. [290] in a jet-stirred reactor at 800–1200 K

at 100 and 1000 kPa for different fuel/oxygen equivalent

ratios from 0.2 to 2.0. Experiments clearly indicate

the different oxidation behaviour of the two isomers

with allene being more reactive than propyne at 1000 kPa

and producing more benzene and other hydrocarbons

except for acetylene. No O-containing compounds were

detected save for CO, CO2 and H2CO. Critical reactions

are presented and discussed together with extended

comparisons of model predictions with their experiments,

with Princeton turbulent flow reactor data for propyne

[287], with higher temperature shock-tube experiments

[286], and with species profiles in an allene-doped fuel-

rich acetylene premixed flame [291]. Isomerization

reactions proceeding via direct and H addition routes are

significant in the oxidation. As a result of H-abstraction

reactions, both propyne and allene form the resonance-

stabilized propargyl radical. These species are important

intermediates in all combustion processes, and their

successive reactions are relevant candidates in explaining

the formation of aromatic and polyaromatic species,

possible precursors of particulate and soot.

Wang [246] has explored the importance of carbenes as

free-radical chain initiators in the oxidation of propyne and

allene (and ethylene) by combined DFT calculations

and kinetic modelling of ignition delay measurements

[286] and concludes that a carbene pathway dominates the

initial radical pool production. Thus for allene, which

isomerises rapidly to propyne, the route is

H2CyCyCH2 !M

H3C–CxCH!O2

½H3C–CHy €C· · ·O2�

! free radicals

Fig. 11. Propyne, allene, butadiene and acetylene; data from Ref. [275].


with the formation of H3Cz þ CzHO þ CO as the likely

outcome. His proposed mechanism is a very good fit to the

allene and propyne data, and, quite good for the ethylene

results.

4.3. Butynes

An experimental and modelling study of the high-

temperature oxidation of both butynes has been carried out

from 1100 to 1600 K at pressures of 630–9100 kPa and for

equivalence ratios of 0.5–2 by Belmekki et al. [292]; they

find that 1-butyne is more reactive than 2-butyne and attribute

the difference to the easier formation of HCxC–CzxH2

radicals from 1 than CH3–CxCz from 2. Their mechanism is

also used to simulate thermal decomposition species profiles

obtained in shock waves by Hidaka et al. for 1-butyne [293]

and 2-butyne [294] with reasonable agreement.

4.4. Diynes

A shockwave and modelling study of diacetylene,

HCxCCxCH, oxidation and pyrolysis by Hidaka et al.

[295] was carried out from 1100 to 2000 K and 110–

260 kPa for mixtures with 0:5 # f # 2: Species profiles

were obtained for the parent itself, ethyne, CO, CO2 and H2

for reaction times of ,2 ms as well as induction times at a

constant initial (preincident shock) pressure of 6670 Pa. The

mechanism of 466 reactions involving 83 species which

could be condensed down to 174 reactions of 51 species

accounted reasonably well for their data.

5. Aromatics

The modelling of aromatic compounds present special

difficulties and kinetic complexities beyond those of most

other hydrocarbons; for example, modelling of benzene

oxidation is inextricably linked with the formation of higher

polycyclic aromatic hydrocarbons and soot [12,13]. Some of

the work in this area has been described, in a telling phrase,

as ‘an extensive literature of bad assumptions’ caused no

doubt by the paucity of experimental data and the lack of

theoretical models to guide the researcher.

5.1. Benzene

Zhang and McKinnon [296] have developed an elemen-

tary reaction mechanism containing 514 reactions without

adjusted parameters for the low-pressure flaming rich

combustion of benzene. Key features of the mechanism

are accounting for pressure-dependent unimolecular and

bimolecular (chemically activated) reactions using QRRK,

inclusion of singlet methylene chemistry, and phenyl radical

oxidation and pyrolysis reactions. The results are compared

to earlier detailed molecule and free-radical profiles

measured using a molecular beam mass spectrometer by

Bittner and Howard [297]. In general, the mechanism does a

good job of predicting stable species and free-radical

profiles in the flame. The computed profiles of small free

radicals, such as H-atom or OzH, match the data quite well.

The largest discrepancies between the model and exper-

iment are phenyl radical and phenoxy radical

concentrations.

A detailed comprehensive kinetic mechanism has been

developed through reaction path flux and sensitivity

analysis by Tan and Frank [298] to model species

profiles in a rich, near-sooting benzene–oxygen–argon

flame [297], speeds of freely propagating benzene–

oxygen–nitrogen flames [299] and ignition delays of a

mixture of 1.69% benzene þ 12.7% oxygen with the

balance argon [300]. Although generally speaking good

agreement was obtained, the reactions of C5 species are

not satisfactory.

Benzene–air flame speeds were measured by Davis et al.

[301] in an atmospheric pressure counterflow flame for

0:8 # f # 1:4 and compared to predictions from mechan-

isms due to Emdee et al. [302] and Lindstedt and Skevis

[303]; a modified version of the Emdee mechanism not only

gave good agreement with the flame speeds but also retained

the ability to give a good fit to atmospheric pressure flow

reactor data for benzene oxidation at temperatures of 1000–

1200 K [302].

Benzene oxidation at two equivalence ratios (f ¼ 0:19

and 1.02) was studied by Chai and Pfefferle in a well-

mixed reactor with a 50 ms mean residence time at

350 Torr and 900–1300 K [304]. Acetylene was the major

hydrocarbon intermediate for both stoichiometries with

phenol (C6H5OH) and acrolein (H2CyCH–CHO) reaching

significant concentrations for the lean condition, while at

the stoichiometric condition C4H4 is more than twice as

abundant compared to the lean case, and phenol and

acrolein are minor intermediates. The predominant radical

species for both conditions is cyclopentadienyl while

cyclopentadienonyl and phenoxy (C6H5Oz) are the next

most abundant radical species detected in the lean

condition.

Marinov et al. [110] studied the chemical structure of an

opposed-flow methane diffusion flame operated in such

a way as to highlight the routes to mono and polycyclic

aromatic hydrocarbons and modelled the results with a

methane mechanism of 156 species participating in 680

reactions. They found good agreement for the large

hydrocarbon aliphatic compounds, aromatics (benzene,

toluene, phenylacetylene, styrene), two- and three-ring

polycyclics (naphthalene, acenaphthalene, phenanthrene,

anthracene) but not with four-membered rings (pyrene and

fluoranthene).

Alzueta et al. [305] have carried out an experimental

study of benzene oxidation in a plug-flow reactor at 900–

1450 K and residence times of ,150 ms; they reacted

107 ppm of benzene with between 830 and 491,000 ppm of


O2 in the presence of 0.5–2% H2O. They compared their

detailed kinetic model to data from turbulent flow [306] and

jet-stirred [304] reactors studies (but not to any flame data),

and, conclude that the flow and stirred reactor data are

incompatible.

Schobel [307] has studied the oxidation of benzene in a

plug-flow reactor at intermediate temperatures, 850–960 K,

and at atmospheric pressure with a view to understanding

the fate of benzene in the burnout zones of waste

incinerators. He measured species profiles as a function of

residence time, temperature and oxygen concentration and

compared the results with simulations based on detailed

models from Zhang and McKinnon [296], Emdee et al.

[302] and Zhong and Bozzelli [308]. The generally poor

agreement with these existing models necessitated extensive

modifications to the Zhang and McKinnon model which

then gave satisfactory agreement.

Ristori et al. [309] have studied benzene oxidation in a

jet-stirred reactor at atmospheric pressure from 1010 to

1295 K for 0:3 # f # 2: They have also modelled shock

tube and flame experiments [301], well-mixed reactor data

[304] and their own perfectly stirred reactor results at

1000 kPa. The mechanism is a particularly good fit to the

benzene–air flame speed data and it performs reasonably

well for other simulations thus laying the foundations for

modelling the kinetics of more complex aromatics.

Sensitivity analysis reveals that reactions of the cyclopenta-

dienyl, phenyl and phenoxy radicals are important, Fig. 12.

Lindstedt et al. [310] highlight some current issues in the

formation and oxidation of aromatics in the context of

detailed kinetic modelling of benzene and butadiene flames

and stirred reactors featuring ethylene and mixed aromatic–

ethylene–hydrogen fuels. In particular, uncertainties per-

taining to the rates and product distributions of a range of

possible naphthalene and indene formation sequences are

discussed from the basis of improved predictions of key

intermediates. The naphthalene formation paths considered

include initiation via cyclopentadienyl radicals, phenyl þ

vinylacetylene, and benzyl þ propargyl recombination. It is

shown that a number of possible formation channels are

plausible and that their relative importance is strongly

dependent upon oxidation conditions. Particular emphasis is

placed on the investigation of formation paths leading to

isomeric indene, C9H8, structures. The latter are typically

ignored despite measured concentrations similar to those of

naphthalene. The rates of formation of C9H8 compounds are

consistent with sequences initiated by reactions of phenyl

with propargyl, C6H5 þ C3H3, and with allene,

C6H5 þ C3H4, leading to indene through repeated isomer-

isation reactions. The current work also shows that reactions

of the indenyl radical with methyl and with triplet

methylene provide a mass growth source that link five-

and six-member ring structures.

Richter and Howard in a very long paper discuss the

formation and consumption of single-ring aromatic hydro-

carbons and their precursors in premixed laminar benzene,

acetylene and ethene low-pressure flames [311] with the

predictive ability of their detailed model [312] ranging from

excellent to fair for the consumption of reactants, formation

of major combustion products and formation/destruction of

intermediates. Self-combination of propargyl, H2CyCyCzH,

followed by ring closure and rearrangement is shown to be

the dominant route for benzene formation in rich acetylene

and ethylene flames whilst phenoxy radical overprediction

is still a problem in determining the fate of phenyl radical

oxidation.

5.2. Other aromatics

Although benzene is the archetypal aromatic, work on

toluene and on other aromatics is probably more represen-

tative of the chemistry that will be encountered in the study

of real fuels, from which of course benzene has been

excluded by regulatory authorities.

A detailed chemical kinetic mechanism for the combus-

tion of toluene has been assembled and evaluated by

Lindstedt and Maurice [313] for a wide range of oxidation

regimes including counterflow diffusion flames [176], plug

flow reactors [314] and premixed flames (more accessible

from [234]), and, for shock-tube pyrolytic experiments [315,

316]. The reaction mechanism features 743 elementary

reactions and 141 species and represents an attempt to

develop a chemical kinetic mechanism applicable to

intermediate and high-temperature oxidation. Toluene

thermal decomposition and radical attack reactions leading

to oxygenated species are given particular attention. The

benzyl radical sub-mechanism is expanded to include

isomerisation and thermal decomposition reactions, which

are important at flame temperatures, and a molecular oxygen

attack path to form the benzylperoxy radical, which is found

to be relevant at lower temperatures. The final toluene

kinetic model results in excellent fuel consumption profiles

in both flames and plug flow reactors and sensible

predictions of the temporal evolution of the hydrogen

radical and pyrolysis products in shock-tube experiments.

The structures of toluene/n-heptane, toluene/n-heptane/

methanol and toluene/methanol diffusion flames are

predicted with reasonable quantitative agreement for

major and minor species profiles. Furthermore, the evol-

ution of major and intermediate species in plug flow reactors

is well modelled and excellent laminar burning velocity

predictions have also been achieved.

A chemical kinetic model was developed by Klotz et al.

[317] to predict the high-temperature oxidation of neatFig. 12. Reactions of phenyl and cyclopentadienyl radicals.


toluene, neat butane, and toluene–butane blends in an

atmospheric-pressure flow reactor at 1170 K. The focus of

this study was on the behaviour of the blended fuel but

extensive validation of the toluene mechanism was also

undertaken during which it emerged that improvements

were needed in the toluene model of Emdee et al. [302].

The changes include addition of iso-butyl reactions, which

significantly improved predictions for 1,3-butadiene and

acetylene. Additionally, improvements were made in the

modelling of benzaldehyde since the experimentally

measured benzaldehyde profiles were obtained with a

gas chromatograph better configured to separate polar

compounds than in previous experimental toluene studies,

and these better profiles led to the adoption of a more

appropriate rate constant for the overall reaction that

accounts for the formation of benzaldehyde during the

oxidation of toluene, C6H5CzH2 þ HOz

2 ! C6H5CHO þ

OzH þ Hz. The modelling results presented demonstrate

that when the chemical interactions between the various

fuel components are limited to radical pool effects, the

blended fuel oxidation process is more likely to be

predicted when the blend model is properly configured to

predict the oxidation processes of the neat fuel

components.

The oxidation of toluene at high dilution in nitrogen was

studied in a jet-stirred reactor at 100 kPa by Dagaut and co-

workers [318] over the temperature range 1000–1375 K,

70–120 ms residence times and variable equivalence ratios,

0:5 # f # 1:5: Concentration profiles of reactants, stable

intermediates and final products were measured by probe

sampling followed by on-line and off-line GC analyses.

These experiments were modelled using a detailed kinetic

reaction mechanism (120 species and 920 reactions) and,

used to simulate the ignition of toluene–oxygen–argon

mixtures [319] and the burning velocities of toluene–air

mixtures [301,234]; there is good agreement with ignition

delay data and with flame speeds for f # 1:2: Sensitivity

analyses and reaction path analyses, based on species rates

of reaction, were used to interpret the results. The routes

involved in toluene oxidation have been delineated: toluene

oxidation proceeds via the formation of benzyl, by H-atom

abstraction, and the formation of benzene, by H-atom

displacement yielding methyl and benzene; benzyl oxi-

dation yields benzaldehyde, that further reacts yielding

phenyl whereas benzyl thermal decomposition yields

acetylene and cyclopentadienyl; further reactions of cyclo-

pentadienyl yield vinylacetylene.

Shock-tube measurements of species profiles obtained in

toluene/oxygen/argon mixtures (f ¼ 1 and 5) at very high

pressure, over 60 MPa, from 1250 to 1450 K have been

made by Sivaramakrishnan et al. [320] and modelled against

the mechanisms of Klotz et al. [317] and of Dagaut et al.

[318]. The latter model severely underpredicts the rate of

toluene decay as well as the concentrations of benzene and

carbon monoxide so the former mechanism was chosen for

slight modifications; the addition of para-quinone channels,

inter alia, improved the fit:

C6H5Oz þ O ! OC6H4O þ Hz

In the case of propylbenzene [321] concentration profiles

for 23 species were obtained at atmospheric pressure over

the temperature range 900–1250 K for 70 ms dwell time

and at three stoichiometries (0.5, 1.0 and 1.5). n-Propyl-

benzene is more reactive than toluene with the production of

ethyl radicals identified as the key early step in the

production of reactive H-atoms:

C6H5CH2CH2CH3 ! C6H5CzH2 þ CzH2CH3

V H2CyCH2 þ Hz

The low-temperature oxidation of butylbenzene was

studied at temperatures between 640 and 840 K by

Ribaucour et al. [322] in a rapid compression machine.

They measured delay times of one- and two-stage autoigni-

tions and intermediate species concentrations after the cool

flame and modelled the results with a mechanism of 1149

reaction and 197 species.

Autoignition data for 11 alkylbenzenes was collected by

Roubaud et al. [323] from 600 to 900 K and at compressed

gas pressures up to 2500 kPa. Toluene, m- and p-xylenes

and 1,3,5-trimethylbenzene ignite only above 900 K and

1600 kPa whilst o-xylene, ethyl, propyl and n-butylben-

zenes, 1,2,3- and 1,2,4-trimethylbenzenes and 2-ethyl-

toluene ignite at much lower temperatures and pressures.

A more detailed study by the same group [324] concentrated

on o-xylene, o-ethyltoluene and n-butylbenzene and

obtained samples of the intermediates formed at 10, 20

and 35% fuel consumption, respectively.

Work on 1-methylnaphthalene by Pitsch [325] and

Shaddix et al. [326] is indicative of the complexity that

awaits when multi-cyclic aromatics are tackled—a daunting

prospect.

6. Conclusions

The ultimate goal of chemical kinetic modelling is to

develop an ideal set of thermodynamic data and a

‘perfect’ reaction mechanism which will describe all the

essential details of the physical reality, specifically the

combustion of a hydrocarbon in the gas-phase. Some

sense of how far we have progressed can be gleaned

from the preceding text.

A series of cooperative efforts are required to progress

the field, unless there are some dramatic developments

on the theoretical side which enable both the calculation

of reaction pathways and rate coefficients of individual

reactions with reasonable precision. Great strides have

been made of late in the computation of rate constants (a

masterly summary by Wagner [327] on the challenges of

combustion for chemical theory is available) but it can

be an acronymic nightmare for the unwary. Since


individual rate constants, k; are required as functions of

temperature and of pressure, k ¼ f ðT ; pÞ; this adds

substantially to the burden. Initiatives such as CSEO

[328] which provide, inter alia, online computations of

rate constants are most welcome and are a signpost for

the future.

On the experimental side a number of different

approaches are required including the measurement of a

few selected critically important rate coefficients, the

measurement of concentration profiles (not just for stable

species but also for such species as OzH, Hz, HOz

2, etc.) in

flow reactors, shock waves and burners, the determination of

complex parameters such as laminar flame velocities,

ignition delay times, et cetera. Since this variety of

experimental techniques is rarely, if ever, available in one

laboratory, it reinforces the notion that co-operative

research is essential.

Once all of this hard-won data has been gathered it must

be properly accessible. A substantial effort is required to

render known data into more useful formats which will

eliminate the problem of nomenclature, encourage data

mining techniques, enhance portability and reduce wheel re-

invention. In particular the archiving of data needs to be

made more rigorous; many of the links mentioned here have

very short time spans.

Finally, modelling just your own experiments with your

own mechanism is scientifically worth very little (unless

there is no other data of course) and one would hope that

journal editors and referees would discourage researchers

from such excessive introspection.

Acknowledgements

I thank my colleague, Henry Curran, for stimulating

discussions and reviewers from this journal for critical input

and a lovely turn of phrase. The assistance of Sheila

Gallagher is gratefully acknowledged.

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